e36 small case wont fit an e30, it has a completely different casing... you could swap over the internals to your small case e30 diff casing, but 3.45 would be a bit too long behind an e30 gearbox. Or just swap in the LSD unit to your 4.1.
An e30 medium case will bolt straight in. the mounting is the same as the small case. Its not like e36s where you need to change shafts and prop
So, if it was me, I'd definitely go for the 3.64 diff
Assuming this is an e30 diff ya?