[spacemanspif]

Not sure on the manual but there is probably adapter bellhousings to put pretty much any trans behind any engine. Switching from gas to diesel is not a run-of-the-mill engine swap and you may need to consider changing rear gears. Usually 3spd cars came with numerically low gears because 3rd is 1:1. However, old diesels rev to ~3500RPM, right? if you have 3.08:1 rear gears and keep your 3spd trans you may run the diesel too high on the highway to get the MPG benefit; pretty much requiring a trans with another gear (or 2, but 6speeds are expensive), or a rear gear change.