12A vs 13B diffs?
 

Alright so went shopping for a new diffrential for my racecar (12A) and found the local junkyard had an 85 with an LSD unfortunatly someone had snagged the engine and almost the entire contents of the engine bay so had no way of knowing if 12A or 13B. However semi idiotic clerk said any model would fit so figured Id go ahead and pull it and see (30 day money back guarntee). However now that Ive gotten a chance to look up the VIN looks like it was a 13B, any diffrences between a 12A LSD and a 13B LSD?

It depends on what year rear end you're using. 84-85 12A & 13B LSD is interchangable. The 13B one has one more plate to make the engagement smoother. 79-83 small axle cars 84-85 have larger axles. 84-88 NA cars are all interchangeable LSD's.

Any easy way to tell which axles you have? Pretty sure my entire rear end (axles, diff, brakes, etc) is from a diffrent car so....

Also is the rear end ratio the same or did the 13B run a diffrent ratio?

13B have 114.3 mm stud spacing all 12a cars had 110 mm if you snagged the whole rear end.