PK numbers

As for the Torque Convertor, It is kindof a magical device. It does two things, maybe three, depending on how you count,lol.
1) the TC provides a fluid coupling so you don't have to clutch it.
2) It is a Torque Multiplying device;
in that more torque comes out the back, than what actually goes in the front, and
3) it can be tuned to transmit the bulk of the engine torque at almost any rpm.

As for #2) most TCs have an finitely variable automatically adjusting multiplier between about 1.8 and 1.1. The multiplier is usually greatest at Zero mph, (not always) and diminishes with both rpm (torque input) and roadspeed (resistance to forward motion).
So lets say you had a TC that stalls at 2000rpm. That means, when you mash the gas at zero mph, the rpm will jump to ~2000rpm before the truck starts moving. If your Torque peak was at 2000rpm and say it was 70 ftlbs, then it is possible that the TC could multiply that 70 by say 1.6 so say 112 ftlbs.
But say you had a 2800, and that was also your torque peak. And say at 2800 your torque was 90ftlbs. That TC might have a multiplier now of 1.8, so 162 ftlbs leaving. That's an increase of 44.6%, just in the TC.......... Now remember, this is at zero mph.
Lets see what is happening at the tires.
Stock;
Your first gear ratio is 2.45 and your rear gear is 3.21, so into the rear axles, you are looking at :
With the 2000 TC and 3.21s
112 x2.45x3.21= 881 ftlbs
With the 2800 and 3.21s
162x2.45x3.21= 1274 ftlbs, a 44.6% increase
With the 2000TC and 3.91s
112x2.45x3.91= 1073 . See how that is ~22% better than the 3.21s, but is only half the percentage increase of what the 2800TC did. lets try
Both a 2800 and 3.91s I get
162x2.45x3.91= 1552, this is now 1552/881= 76% better than stock! ........ and we never even touched the engine! Remember, this is at zero mph. Remember,if the stall is at the torque-peak, then the multiplier will begin to diminish just as soon as the truck begins to move.
Finally; once the engine is past stall and continuously loaded (as in a dragrace) the multiplier will continuously diminish to it's minimum value, with minor blips along the way during gear changes. Therefore, after ~2800rpm, it no longer matters much what the less than 2800 stall, actually is.
Also bear in mind, I arbitrarily choose the torque numbers in these examples as a best guess; I just wanted you to see the magic that happens with the TC and gears.