Part-1
well, I hate to be the bearer of bad news but;
the 71 340 never had an Scr of 10.25 to 10.57.
It was rated at 10.5 but that was a lie just like it's power-rating of 275 as a 4bbl was a lie.
Do the math;
The pop-up was NOT .045., maybe 015 for -3cc
The gaskets were .020, maybe 4.5cc
the open chambers were in the window of 70>72cc
and the 4eyebrows per piston, I'm guessig were ~8cc.
Thus the total chamber volume was about 81.5cc
the swept was 696.4, so
the Scr was....... 9.55, at best! with 70cc heads.
The math does not lie.
65cc chambers and 054 gaskets will change the total chamber volume from 81.5 to 78.6, for a new Scr of 9.86.
But, listen; the engine could NOT care less about it's Scr.
What it cares about are Squish(commonly called Quench) and cylinder pressure, We use Dcr and fuel octane, to guide us to the required Scr to achieve the safe working pressure.
In your case, with the Scr already established by the chosen parts, now we have to work the number backwards to arrive at an acceptable pressure. But doing that, requires knowing the camshaft timing events, and the installed position; which you did not supply.
So if you are building this for the street; IMO, you're in a bit of a pickle.
Part-2
Your Dakota estimated weight of 3900, is, IMO very conservative. But say you get it down to that weight; this is ~15% heavier than the heaviest A-body, and ~26% heavier than the 67>69 A's, plus Dusters, Demons, and 2-dr Scamps .
To compete with their weight advantage, you would need Big-Block power, which the 340 can do using rpm. But at a common stall speed, the only way to compete is with the same 26% more overall gearing. or by up-stalling to get the same delivered ftlbs.
Part-3
Next you have the Tire size differential.
The original 340 A's had E70-15s on them, at about 24.5 inches tall.
Your Dakota came with 235/75-15s at a height of about 28.9 inches.
This is a difference of 17.9%. Which means, you're gonna need that much more gearing to compare, so your 2.92s compare to 2.48s in an A-body. Dog city.
To compare to 3.55s you would need 4.19s
Part-4
Stall.
Ima thinking your cam is about a 262 to probably a 268. and the Ica will come in at around 64*. If I'm right, the P/V, which is a performance ratio of Pressure and cylinder Volume, is gonna come in around 126 with a pressure of around 157psi, at 500ft elevation.
Are you sitting down?
At low rpm, with the equivalent of 4.19 gears, under say 3500rpm, the V/P is estimating that your Hot-Rod 340 will feel about as powerful as a 5.2 Magnum, assuming they both power thru the same Convertor. Therefore, if you want more take off performance, yur gonna have swap out your low stall TC...... or go even deeper in the rear gears.
Part-5
All these other changes are costing you time and money. Hi-stall convertors are not cheep. New gears installed are not cheap. Wheels and tires are not cheap.
But to me, the biggest travesty is running alloy heads at a paltry 9.5 Scr.
The combination of ALLOY heads and a Tight-squish in my 360LA, have easily run at, up to 195psi, still burning 87E10, since 1999.
By you not going there, you are leaving a lot of power under the table.
If you did that you could get away with a lot less gearing and stall.
Part-6
But if I had a 340, in a Dakota, I would just run an A518 4-speed auto...... with mega gears in the back. and I would run a cam of no more than about a 223/110 @050 cam, (or say a 228/108 cam) so I could gear it at 65=no more than 2400, so I can cruise the beast. In this case, I might consider, just for kicks, a small Whiplash or Thumpr. I said "might", lol.
