Since you have a TH350 already, the 700R4 is probably the best overdrive
candidate for you. They aren't cheap (good ones, that is). You want
one from the late 1980s, but you have to make sure you can connect a
speedometer to it, as these later ones often had electronic speedos.
The earlier trannys had a smaller input shaft, as well as many other
problems that got fixed by GM over the years. The installation will be
relatively easy, you'll have to move the rear tranny crossmember back
two inches, and shorten the driveshaft (unless you have a long tailshaft
TH350). The detent cable is critical on the 700, since it has no vacuum
modulator to tell it when to shift. You will need a 700 dipstick and
tube. You'll also want to wire up the lockup torque converter clutch,
there are kits available that make it lock up as it shifts into
overdrive. Since you say that you're not a mechanic, you may not quite
understand what all this means...so ask more questions, and do some
research. We'll be glad to help you learn!
A rebuilt 700 costs from $600 to $1500, and good used ones ("good" has
many meanings) usually run $250 and up.
Personally, I prefer a big engine and mild gears, such as a 454 with a
3.0 rearend and a 3 speed automatic.
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959