'70 Chrysler 300 8 3/4???

look for casting numbers on the center chunk.
they will either be 741, 742 or 489
I personally like the 489 chunks, but the 742 chunk is great as well

If narrowing the housing for an A body the axles may be able to be re-splined.

Quick google search this information is a good read for your questions.
http://members.tripod.com/mojo_page/chry875.htm

I pull alot from cars in junk yards for 100 bucks. Alot only have 2.73 gears and open.