727 Difference inside

What AbodyBB is talking about, is the front planetary splines to the front output splines. A lot of guys changed from a 3 pinion to a 4 pinion planetary and did not match the splines. It would work ok for a short time, then rip the splines right out of the planetary or break it into a couple of parts. You will know that it is wrong match if you try to wiggle the planetary on the output shaft and it acts like it may be worn a little. It has to fit tight. Ask me how I know.