Will it interchange

Personally I think this is a bad idea. I would bet there's a lot more need for A body 4 speeds, but I could be wrong.

At least some A body boxes could be converted with the correct mainshaft and housing

But the bell only fits smallblocks.

Also there are different retainer sizes, depending. So if he picks up a bell that happened to have the hole for the big 18 spline box, the retainer hole in the bell will be too large

What he needs is an OD out of a 2x4 van or pickup