Look at the outline of the pan on the transmission. There should be a "bump" on the passenger side near the fill tube. Just enough where it won't look square ish. It's a 727. If it is straight in this area it's a 904. That being said, all big block autos in the muscle car era were 727's. Some people modify 904's for drag racing since they shift faster so depending on your cars past life you just may have one.
