Factory performance 340/360 heads are very similar/same with, to your 318, no difference. These heads carried a cast-in letter designation on the flat part between the sparkplugs. The letters are one of the following; X, J, U, and O.
These are all large open chamber heads. Some have 2.02/1.60 valves and some have 1.88/1.50 valves. Most are hardened seats for use with unleaded gas .......... but not all.
The 73-318 engine is a low compression model rated at 8.1/1 but was usually quite a bit less. This only becomes a problem as the engine moves into a higher elevation, or until you bolt on heads with bigger chambers, making the already low compression ratio, even lower.
In your case going from the small-port 318 heads to the large-port 340/360 heads brings a couple of very un-welcome additional ccs, that will do nothing for you except make your engine sluggish below 3000 rpm, and lazy on the up-take. I highly recommend you stay with the fresh heads you already have.
As for the intake,
while the 340/360 large-port intakes bolt-on and work, if you are replacing a smallport with them, you will immediately feel the difference, and not in a good way. If you already have one, you can try it. But if you have to buy an intake, then I highly recommend to get a small-port intake. And make it a dual-plane, to pick up the low-rpm torque.
As for carbs, on 318s I prefer spread-bores with tiny primaries. With stock cam, any small cfm will do, but metering rod carbs might offer better steady-state fuel-economy.
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But before you do anything,
and if it is a more performance-like take-off that you are after, then;
if you have sub 3-series gears, and your engine cannot spin the tires off the line with the two barrel, then it will not get any better with a 4-barrel. And after the gear-swap, if it still cannot spin the tires, then you will need a higher stall Convertor. These need to be done first,
With that done,
now is the time to install the 4bbl together with a free-flowing exhaust.
Here's the why of it;
Typically, 318 A-body grocery-getter cars came with 2.76ish gears.
Typically they came with a stall from 1700 to 2200..
Typically the 8/1 engines had very little torque at those stalls.
And the 904 has ratios of 2.45-1.45-1.00.
At the bottom of this post is a 5.2 powercurve. Your 318LA will not look like this, but I just want you to see the low-rpm torque as a reference.
On this graph, at 1800 you will see about 270 ftlbs NET at the crank. By the time this gets to your rear axle, it will have been multiplied to
270 x 2.45 x say 2.76 rear gear =
1830 ftlbs@1800rpm. This is pretty abysmal. Since your 4-bbl will not even be in the least bit encouraged to open@2200, that means it doesn't make a single bit of difference if you bolt it on.
Now lets go to 2800; now the torque of this MAGNUM has risen to over 290, say
293. Now, at the rear this will be multiplied to
293 x2.45 x2.76=
1980, and at 2800 the 4bbl could be wide open now
But these numbers are 5.2M numbers; your LA will be lucky to pull 80% of that, even with the 4 bbl at wide open; so let's say 1980 x.8=
1585; still too low to spin BOTH tires.
Ok but, the TC has a multiplier of perhaps 1.8 at zero mph ,diminishing to 1.2 over time and distance, so;
1585 x 1.8 =2853. this is enough to break them loose, but as soon as that happens, the multiplier starts to choke up and a few feet out, the ratio may have dropped to say 1.3, so 1585 x 1.3=
2060 and the spinning has stopped. Remember, the 4bbl is wide open.
Ok so what is the cure?
Well, by my experience, I need about 2700 ftlbs to spin a pair of 255s. So
2700/2060= plus 31% more torque in the middle of first gear. that would then be
2.76 x1.31=3.62s, rounds to 3.55 reargears.
Ok so lets run that thru the math-grinder;
293 times .8 to convert to LA x2.45 x3.55 x1.3 =2650, close enough.
Ok now, let's take the primaries away; say we net 200ftlbs at 2800, that would be;
200 x2.45 x3.55 x1.3 at the top of first gear =2260, but
200 x2.45 x3.55 x1.8=3130 at zero mph, plenty enough to start the spin and to continue until the TC ratio drops some; say to; 3000/(2.45 x3.55 x say 240ftlbs at peak) =1.44 ratio so that is most of the way thru first gear.
The point being that, to get you moving, and most if not all the way thru first gear; the Stall and 3.55s are all you need; and the 4bbl just creates additional tirespin. You won't be needing the secondaries until well after 3000rpm, I'll guess closer to 3600. IMO, they will be more of a Second-gear deal at this point.
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Finally, I gotta tell you that, this is the exact method I applied to my 68 Barracuda in year 2000 when I was engineering my winter-engine. I freshened my smoggerteen, gave it a 2800TC and 3.55s , the headers stayed in the chassis when I took the 367 out, and so did the 3x3 dual pipes. This was a dynomite package.
A few weeks or months later, I lucked into a small-port iron intake, installed that with a Thermoquad spreadbore carb. Not long after, I found this package TOO powerful for winter use, so first 3.23s went in, then 2.76s,
I used to swap that smoggerteen into the Barracuda, every September long weekend, and swap the 367 back in on May-long. Five or six times in a row. The teener rarely got the same trans and gears two winters in a row, and the 367 also got a bunch of different combos; but always a manual trans. Yes that was a lot of work, but that is now two decades ago.
As for the 318, if it was backed by an automatic, it always had the 2800TC. Nice unit. Sometimes I used the A904 sometimes an A998 non-LU. But the gear of choice eventually settled at 2.76s, with the A998 trans.
This provided plenty of First-gear Giddyup in winter; And second was not that bad; 2.76 x1.54=4.25, compared to; 4.25/1.45=2.93s with the standard A904. The key to happiness was the 2800TC and the small Spreadbore carb.
Oh and Ignition timing.
Oh and maybe the headers mighta had something to do with it, but with stock heads and cam, I sorta doubt it. But yeah, the 3x3 dual pipes were worth something,lol.
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So bottom line is this;
if the tires don't spin, then
the 4bbl will do NOTHING at take-off until the secondaries open, which could be as late as possibly30 mph or as early as 20, depending on the engine's willingness to take throttle, and your ability to tune it. At the time they are full open, lets say 2800, suppose your 318 picks up 16ftlbs. That would be 16x2800/5250=
8.5 hp@2800.
Now suppose your 318 makes 220 ftlbs at 2200 stall with a 2bbl, and suppose it makes 236@2800, still with the 2bbl; this is a difference of 16ftlbs and;
16 x 2800/5250=
8.5hp@2800, still with the 2bbl...... same/same right?
Well um ,no; not same same.
In the first example you are taking off at 2200 which is 220 ftlbs and this will slowly grow to 236@2800, which could be say 25mph. whereas
In the second example; you are taking off at already 236 ftlbs , and by 25mph, your rpm could be 3200, and the engine making 240 ftlbs .
So, for take off power, in this example, your combo, the higher stall trumps the 4bbl, all the way to 25 mph at least, maybe 30,....... with 2.76 type gears.
If you have bigger gears the speed range will decrease by the percentage difference in the gears. Bigger gears will allow the engine to blast up into the power-band quicker, and then also run thru the gears quicker, thus putting your horsepower down in a shorter amount of time. That is generally called a greater average power over a measured distance and thus becomes a faster rate of acceleration.
Now, you may not be looking for a faster rate of acceleration, nor the ability to spin your tires for hundreds of feet; IDK, you didn't say.
Maybe you are just looking for better passing power? And want the Cruise-Rpm to stay where it is. Well; Now the 4bbl move is probably the best answer. Cuz 55 in Second gear with 2.76s, is a measeley 2900 and the 4bbl is at WOT, so about plus 9 hp or something. Doesn't sound like much. (but if you only have 140@2900, then 9 is 6.4% so that's something.)
5.2Magnum
