Hard to change the tranny without considering the rest of the driveline...
I don't have any specific knowledge (i.e. is it a direct bolt on). But you might be better off with a 4-spd from the 70's era - it will be better matched to the engine, and easier to install. Cheaper too, unless you already have one of them 5-speeds. Maybe still cheaper, if special adapter plates are required.
The top gear is going to be 1:1 for all trannys, unless there's an overdrive involved. So no change in top speed, just more shifts from 0 to 60. You'll have great torque (and much stronger leg and arm muscles) but no real drivability benefit.
If you were going to swap out the rear end for a higher gearing at the same time, then it makes more sense to go for the 5-speed. You'd get higher top speed, and similar power for the lower gears.
Otherwise the 4-speed is a better swap, and can probably be done in a weekend or three.
You will likely have to lengthen or shorten the driveshaft for any swap. Drive shaft shops can do this, probably $150 or so.
-Boyo
