Questions from a newbie...

Fellas, first off sorry for these elementary questions but I'm brand new to Mopar. I'm a Ford and Pontiac owner, but don't hold that against me.

I recently bought a '70 Duster, base car (6 cyl, bench seat, etc.). I'm going to take her down to the bones and I'd like to build a 4 speed car. I'm looking at A833 trans and I keep seeing them listed as a B body or A body. Is there a difference in the tail shaft or some other component that won't allow it to work with any car from the model year? Do I have to find an A body trans or will any A833 work?

Finally, I still haven't decided between a 340, 383, or 440. Does the spline count matter for small block vs big block? I've seen 18 spline and 23 spline.

Thanks in advance for the help.