Slant 6 doesn’t go back in

Not sure anyone explained...........the crankshaft register was made larger around 68ish? So with an early crankshaft you MUST have an early converter, or one (somewhat expensive) specially made with the early, small snout. Depending on your situation/ engine combo, you could consider finding a later crankshaft, engine, or somewhat bad engine cheap to get the later crankshaft.

"The other way around" that is, putting an early (small) converter snout into a later, larger crank register, there used to be a bushing ring you could buy for that. Those would not be too difficult to build........a lathe.