You need to determine ring gear diameter / case volume or the diff you have in the car. Once you do that you can search other cars. Some common ring gear diameters are 185 mm 196 ( 195? ) 210 . There are various case volumes ( this is the amount of oil used ) and sometimes a reinforced case.

Next you need to figure out what axle length will work with your track width and diff. There is a 124 performance site that has a few pages of diff numbers and interchanges but it isn't complete.