What rearend I have?

Hey guys! I hope some of you in here will be able to help me out with this question. I have a 1966 Pontiac Lemans that is a mixed breed. It has a big block chevy and TH400 tranny. My question is about the rearend. I need to know what to look for in order to get some lower gears for it. It has 3:08 in it now, and I want to put in 3:73 or 3:90. I need to know if there are casting numbers on the outside of the housing I can check to see what carrier I have in order to get the gears. I know it is a 12 bolt, but dont know if it the original. I also know it has a spool in it from the way it drives. If someone could help I would appreciate it. Thanks!