as a old hotrodder purists make me cringe
esp in non collector cars like 300sd

swap in a chevy 6 speed and it will go fine both at low speed and hiway
with the right rear end gears
adpter plates are not hard to make or just custom do the whole bell housing
with any bolt patern you want on eathor end
why hunt for over priced rare parts when cheaper stronger stuff is in junk yards
the trans does not care what fuel is burned to make the power
a 300hp rated chevy will be fine in a 5 or 6 cyl diesel