You are in a bit of trouble.
Firstly
If you are getting into top gear with 3.23s then yes your stock 340, in a Duster, is beginning to feel it's small block limitations. This will not change, with any transmission you can name. All you can do for more performance in top gear, is use a higher number rear gear. but if you then run out of rpm, your done, so;
you need more motor. The stock 340, was always done by 5500 or by 100mph or so with 3.23s , whichever came first, and the low compression versions, just took longer to get there.
>So, to recap, if you are really running into a lack of power in top gear, is that you need more power, or more gear.
At 5500 @10% slip (pretty standard with a stock convertor);
3.23s will get you about 123mph
3.73s about 107
3.91s about 102
the trade-off, of course, is cruising at higher rpms.
Automatics with modest stalls will usually get you to a hundred a lil quicker than a 4-speed. and that's mostly cuz of the torque multiplication inside the convertor, which is like a two-speed with a gear ratio that is infinitely variable from somewhere between 1.0 and 2.0 and it is torque sensitive and shifts automatically; thus making your 3-speed automatic transmission, actually work like a four speed manual, except there is no power interruption on the shift.
Stick with an automatic.
If you are spinning the tires all thru first gear, with say 3.91s, you gotta fix that first, cuz you are losing at least half a second right there.
If you cannot spin all thru first gear with 3.91s, before traction work, yur motor is weak.
How much stall to choose, will depend on your engine torque characteristics; and if you change the torque characteristics then you will likely need a new TC. again.
Cylinder pressure makes heat, which is power. If you want to make power with a small engine, yur gonna need pressure and rpm. and then yur gonna have to figure out how to not blow it up.
Gear for gear, I recommend to not race any engines bigger than yur own, at your same weight. and never start in a gear that burns rubber, lol.
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In High school,
I had a nearly new, 70 Swinger 340 4-speed/3.55s, all stock. It went 98 in the quarter at 14.4 seconds, burning rubber all the way thru two gears at least. So it was just yur average 340. But, it never lost a race to anybody else in hi-school.
My secret was to never start from a dead stop, and to never race past 85 mph. I knew the limitations of my engine, and of that Dart. From 30 to 85 it was a monster. But by 100, it was already laying down.
It didn't take long before I installed 4.10s, slapper bars, bigger tires, and lowered my cruizing speed. Now I hunted motorcycles. small ones, lol. Cuz even a good 350 could walk away from me from a dead stop in first gear. But it was always fun to play catch-up in Second gear. I still remember this one Elsinore ........... . We were stopped at a light. The road ahead banked right at ~45 degrees, opened up to three lanes, and went downhill underneath an underpass. When the green came we were on it! but that Elsinore was on the back tire, and carrying the front wheel like nothing! And I was desperately trying to catch him. I finally did, close to the bottom of the hill, but I was deep into Second gear that's for sure, lol.
We pulled over and chatted a bit. It turns out, I knew the guy, and he was two grades ahead of me, but in a different school. Top of second gear with 3.55s was 65mph. So I learned a lil something that day.
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If you really want to run a stick, run a really close-ratio one, so the engine stays on the cam.
My 367 runs a Commando and a GVod behind it, and I split gears. With 3.55s, the roadgears are;
10.97-8.56-6.83-(5.32/4.97)-(3.88-3.55)-2.77 GV in red. yes 8 ratios, but look closely, two of them are too close together to bother with. So really, it's Six ratios.
the splits are
.78-.80-.78-, and .52 from 5.32 into 2.77 ratio
5.32 gets me 93mph@6160 (in the Eighth) by the math; with just one pull on the stick, and two nearly instant electric shifts.
4.97 gets me 93 @5760
I normally shift the first three in the main, then 3od, then 4od; the roadgears are
10.97-6.83-4.97-3.88-2.77, and the splits are
.62-.73-.78-.71 cruizes at 65=2240
IMO, that's a hard progression to beat.
When you go 5-speed shopping, keep these splits in mind.
Oh BTW,
here is what your automatic roadgears look like to the engine. This will be the regular TF with a lock up, and 3.91s
19.15>11.50-6.80-4.69-3.91 in L-up; cruizes at 65=3165
As you can see, 19.15 is ridiculously low, and 3.91s are a lousy cruiser gear.
What you want in a dual purpose automatic street car is something like;
15.82>9.50-5.15-3.55-3.23 in L-up, cruizes at 65=2610; or better yet;
14.40>7.92-4.69-3.23-2.94 in L-up; cruizes at 65=2380
Now you just gotta build an engine to pull that, and may I suggest a small-cammed, alloy-headed, very hi-compression 367, (Like mine, lol); and then stall it until she does what you want it to do off the line, without too much tirespin.
Or; you know, get a stroker, which can pull just about any rear gear, without too much trouble..... like say 2.76s; but you'll be paying a lotta money for to run like one rear end ratio deeper.
You already have 3.23s, and you already have an automatic.
So then, maybe you just need more traction, and/or more inherent torque, and probably a higher stall, and eventually, if the cruise-rpm is too high for you, get a lock-up.
BTW; the 340 can be built for torque too; which is what a dual-purpose car needs. It just takes a more careful selection of parts, a lil more machining, and it costs more than just slamming a 360 together, in the which, they come in really close to zero deck with off-the-shelf pistons, and ~6% more swept are is hard to argue with. Bore the 360 out to 4.04 and call it your 340-stroked to 3.58.lol.
But yeah, you can stick a 3.58 crank into your 340, it just costs more to get to zero-deck.
And, IMO, those X/J/O/U/whatever heads, gotta go anyway, cuz you just cannot run the cylinder pressure up to 195psi, with iron heads.
Why do you want to run 195? Well, if you recall;
pressure makes heat which makes torque, which at rpm, is power. and
195 is 18% more than 165, which for most of us, is where iron heads top out at.
18% is a lot.
It's like two sizes of rear gear; or possibly, up to two sizes of cams. So if you want to run those 2.94s ........... you want to run lots of psi. and if you don't want to rev to 7000, then you want to down-cam, and let the pressure do the work.
A hi-pressure 360 will easily get you into the 12s. Back in the early 70s, a street car that pulls 12s was a pretty big deal, an even bigger deal to a 17 year old hi-school kid. and It's still a big deal to those that can't throw cubic dollars at a project.
BTW-3
Running hi-pressure is not just about the psi. It's also about, running a higher vacuum, and more importantly setting the stage for the overlap cycle, and MOST importantly, it affects how late you can close the intake valve and still have compression pressure.
And lets not forget fuel economy. The more pressure you can run while cruising, the lower a cruise rpm you can run, and the lower the rpm goes, within a good-sized window, the less fuel it's going to burn, saving you money at the pump; money that you can put back into into the hi-pressure combo. If I had to guess, I'd say the fuel savings alone, paid for the cost of my alloy heads running 11.3 Scr, probably in two summers or less. I stole that cash money, from our food budget, but the weekly fuel savings, put most of it back on the table every week over the next year or two. And eventually, I had her running at 65=1650 rpm, and getting absolutely stunning fuel-economy. Yes this 12-second car was my DD for about six years.
Don't let anyone tell you that you don't need hi-pressure; and it pays for itself in the long run.
Now, go do a compression test!, and lets see how bad it really is.
