I really like the ratios of the four speed for track work, but it would be nice to have a five speed for driving to the circuit. Is it possible to fit five speed internals into my four speed case? Or rebuild a five speed with the first four ratios from the four speed?

I will also be turbocharging my 1500, so would it be better to just install a five speed and be done with it?

The 81 X19 I have has a 4 speed in it from a 79. It bolts up and shifts through the gears. That's about all I know, because the engine doesn't run yet. A previous owner broke the 5 speed and let it sit for years until the person I bought it from parted a 79 to restore it then lost interest. It's been sitting in a barn since 1987 acting as a home for an amazing number of mice . I would imagine, if a 4 speed works in a 5 speed car, then the reverse would be true.

I thought I replied to this weeks ago but something must have messed up...

4spd - 5spd is relatively simple, you need the drive shafts and lhs rear swingarm. Possibly a few other bits and pieces.

As to making a big difference... well there isn't that much of a difference. 3rd and 4th are a bit closer while 5th is a bit longer than 4th was and thats about it.

If you want to "convert" your 4-spd box you should be able to do it (never tried myself) but you will need the extended end cover from a 5-spd and as a result you will also need the swingarm to match.