383 Into Stock Slant Six K Member?

You will have more room with a 383-400 B vs a 440 RB, you could add a stroker crank to a 400 and there ya go mega HP, but it is all about your budget.

You can fit what ever you can afford, the 400 is an easier fit to work on for the $$.. Call TTI for exhaust and you need mounts conversion kit is all.