First off, lemmee say that I get where yur coming from. And I don't want talk about the engine, but in your case, I sortof have to, but maybe not for the reasons you might think.
The thing is this; it's all about footpounds from idle to about 2500 rpm. Your cam is a great choice, but you really need to know how much cylinder pressure your 340 is making, before you start throwing gears at it.
Your engine is 50 years old and is still at stock bore. It was an advertised 8.5 engine. Even when new, with the 268/276/114 factory cam, that engine barely cranked 130 psi at sea level. which , in todays terms is atrocious, and makes for a really really soft take of, from idle to around 3000 rpm.
Even if I fudge your cam numbers to 264/264/108 advertised@ zero lash, the Ica is gonna come in at around 56*. And Eastern OK, appears to be around 1000ft elevation.
These two combine to predict a cylinder pressure of 137psi, which is mediocre at best.
What this means is that to get performance out of that engine, it's gonna take a lotta gear Multiplication, and once she starts bucking a wind, with 318 type hiway-gears, in top gear, it's gonna feel like a 318.
I hate to tell you this, but gears will not cure this.
That cam has the potential to make great torque, and great fuel economy, but not at 137psi.
Having said that, lets talk about what you are thinking of doing. Lets try to forget about the engine.
Lets recap;
> you said; The car is slow to get off the line and can't crawl.
> you said; My cam is great. I love it and I'm not blaming the sluggishness on the cam
> and you have a 2.47 gearbox with, 3.21s in the back.
Ok My ears hear, that cam should have a power-peak around 5000rpm. Which translates to a torque-peak around 3500. But with low cylinder pressure , the torque peak may be less and come early, whereas the power-peak may come later and be quite a bit less. Plus with log manifolds the Overlap period does not work. Therefore, your cam will run about on par with a standard 360 2bbl cam (256/262/112, Ica of 52*, except with less pressure, and a lil more power. Oh wait, I promised to try to not talk about the engine.
Ok so, now,
you think your only options are
More torque multiplication.
and that will work, to an extent. But you are, IMO attacking this from the wrong end.
Firstly; First gear is only used to get to Second gear, which for a manual trans car is the gear in which everything happens. If you gear your car right, it will be revved out at ~60 mph, which with your cam should be about 5000>5300.
Letspick 5300.
This will take about 3.91s for 61.5= 5300, with 27" tires, and the current trans. and so half that speed will be half that rpm, 30.75=2650.
Ok so sure you can rev First to 5300 and get 44mph. That's not the point. The point is that if you run less than 3.91s you will not get to use ALL of Second gear, in the run from zero to 60 mph, and your car will be slower than if you had run the 3.91s.
So, here you are trying to run 3.21s; and 5300= 54 in First/75 in Second, and of course it is sluggish, cuz the engine is way down on footpounds in Second gear. OOps. doing just 4250 rpm@60mph, with a dead overlap cycle.
Ok so lets replace that T/A box, and get a Commando.
The ratios are 3.09-1.91-1.39-1.00 With the 3.21s your shift rpm speeds are now 43 and 69, so yur still 15% too high in Second, to overcome sluggishness off the line, she now needs 3.73s for 60=5323rpm.
So no matter how you cut it, yur gonna need BOTH a new gearbox AND rear gears, to overcome the low-pressure handicap, off the line.
If I was in your position, I'd have that engine out in a heartbeat, and build some cylinder pressure into it, which will take about 90% of the sluggishness out of it. And no, notta chance would I reuse those 50 year old bores. Ima thinking 10/1 is/or might be the place to start, it depends on your elevation and ICA. Sadly According to WIKI, 54801 has quite the changes in elevation from one side of the state to the other, so that might be a limiting factor.
I know you don't want to hear it and already gave two guys an earful, but com'on man, there's an elephant in the room and you can't say that nobody told you, I'm just numberr 3 or 4.
What's a replacement gearbox and different gears gonna cost you installed? I bet more than new pistons and a rebore.
BTW-1
If yur having trouble getting that 340 to pull itself at 500 rpm, that there is proof that the tune is off and maybe the engine needs more cylinder pressure. Yeah I get that 500 rpm in your current combo is 5mph and that ain't exactly crawling, but, the 3.09 low will only get you 4mph still not low enough to parade with. Yur gonna need 3.73s to get down to 500= 3.5, or 3.91s to get 550=3.66mph. But if your oilpump can't keep the engine happy down there, then there's no sense in talking about it. So yur gonna have to install a hi-volume pump anyway and a hi-capacity oilpan. I mean pistons are then, only a few more bolts. OOps sorry, lol.
AFTER you have installed the hi-volume pump, I'll give you a recipe to idle at 500 rpm. Oh heck, just retard the stinking timing to 5*, and away it will go. Oh wait, maybe not at 137psi CCP.
By now, you should have figured out that, with a manual trans, EVERYTHING is tied to everything else, and it all has to be working together.
BTW-2
Your Best bet, if keeping the cylinder pressure low, is to get an overdrive five speed, and run the biggest starter gear you dare. Or, in your case if running the 3.21s, then a deep-low 5-speed.
BTW-3
If I was gonna rebuild a 340, I would install flat-tops at near zero deck height, and Alloy closed-chamber heads, for a tight Quench between 030 and 040, and run the cylinder pressure up close to or just over 195 psi. Now yur talking.
Now you can run any box any gears.
BTW-4
notta chance would I try to put the Commando gears into the TA box. Instead, I would put the Commando gears into a A/F alloy box, with the alloy tail, and the alloy cover, and the matching alloy BH. Oh wait, I already did that. Great combo. lol
Oh and notta chance would I run an aluminum flywheel on the street with your combo, so hopefully you got that pig-heavy factory flywheel.
BTW-5
Remember my funky formula of 1600/ starter gear = suggested cylinder pressure?
Well 1600/ 7.9 your current combo = 202 suggested psi
Whereas with the 3.09 low, it is 1600/9.9= 162psi,
Whereas with just 137psi it works out to 1600/137= 11.68.
To get that with a 3.09 low is 11.68/3.09= 3.78 rear gears, rounds down to 3.73s, and 65= 3020 with 27"tires.
I have used this formula with all my personal combos and it works great.
BTW-6
I get what yur trying to do, and your ideas are good, but IMO, just not radical enough for a low-pressure engine.
On another note; if the engine gets jumpy at low rpm, take a bunch of timing out. Bring it down to about 5*, reset the Transfer slot exposure if you need to. Trust me, even my 292/292/108 cam idled at 5*, sometimes even less. If yours doesn't, with adequate valve lash; something is not right.
OTHER
>Your results may vary, and for sure things will change once the headers are on. The H-pipe will add quite a bit of Low-rpm torque, so I wouldn't to anything until after they are on and a new tune is in the engine.
>Listen, my deepest apologies for dragging your engine into the conversation. But I had to. Hopefully you now understand why.
Pressure makes heat. Heat makes torque. Torque is Power. Power is performance.
In the absence of Pressure, you gotta throw Torque-multiplication at it, but when that builds to a head, in top gear, it's back to just motor.
Happy HotRodding.