easyiest way is by the pan on the bottom 727 is some what L shaped and the 904 is more square

 

If they came from behind a big block theyre 727`s, you didnt get a 904 w/ big blocks, if the driveshaft yoke that fits into the back of the trans is a small diameter, will easily fit in your hand its a 904, if it barley fits in your hand its a 727.

 

Measure them from stem to stern, if you are having trouble, the 727 should be 35"

 

That can be debateable. While the 727 has many more performance parts for it, the 904 can be built up just as strong as the 727, it just costs more to do it and the parts are harder to find.