Start checking local shops that do rear-end work. Like most folks say, you need taller (numerically smaller) rear end gears.
A motor swap is do-able, but tough - mounts and bellhousing must change, or else you have to do engine and transmission both. Not a bad idea, but $$$. Do keep an eye out for a wrecked/scrapped mid-50s truck with a V8 setup, but for now, try to get a 3.5-ish or so rear end ratio.
This will cruise nicely, but still pull well with that 4 speed.
Personally, I don't think a T5 would help much. It depends on what's in the axle now. 4.56 rear gears and a .9 OD is still pretty short gearing.....
Check out:
http://www.chevytrucks.org/tech/s10t5.htm Cheers!
George
