Big bolt vs Small bolt
Not sure what is meant by "across 2 studs" but that isn't how you measure a bolt circle. At least I've never heard of it. For the OP, the "big" is 4.50" bolt circle, and the "small" is 4.00" bolt circle, and if you are trying to use your original wheels you will need to get rear axles with the 4.00" bolt circle. You can easily estimate a five-lug bolt circle's size by doing the following:
http://www.roadkillcustoms.com/hot-rods-rat-rods/bolt-pattern-reference.asp#axzz3WFaKGvRy
Here is another way to measure Bolt Circles:
Not sure what is meant by "across 2 studs" but that isn't how you measure a bolt circle. At least I've never heard of it. For the OP, the "big" is 4.50" bolt circle, and the "small" is 4.00" bolt circle, and if you are trying to use your original wheels you will need to get rear axles with the 4.00" bolt circle. You can easily estimate a five-lug bolt circle's size by doing the following:
http://www.roadkillcustoms.com/hot-rods-rat-rods/bolt-pattern-reference.asp#axzz3WFaKGvRy
Here is another way to measure Bolt Circles: