Get ready for a major loss of low-end torque when you go to the dual-carb setup. The only real advantge to going to twice as much carburetion as you need is a little more top-end horsepower and RPM, assuming you ever get the truck off the starting line.
Two things help takeoff, low gears and/or lots of compression. It's possible to mill the cylinder head, but not without a lot of other machining involved. Swapping to a 700R4 automatic will help the gear ratio situation, since it's got as very low-ratio first gear, and the torque converter effectively doubles engine torque at breakaway, just before the wheels start to turn. That will give you a better holeshot with the tall tires, and if you go to a converter with a higher stall speed, the ability to wind the engine up a little tighter on takeoff will help, also. If you insist on sticking with the standard transmission, you'll need to find a lower-geared rear end like a 4.56, and that will drop your top-end speed somewhat. Lower-profile tires are the obvious, and least expensive solution to your takeoff problems.
Jerry
