727 locking up while driving

I just put a 727 in my dart. It has a cheetah full manual valve body. I took it for a drive today for the first time and Tried to put it in second gear and it completely locked up the rear tires. I loosened up the kickdown band and it’s still really tight. If I put it in 2nd gear it brings the car to a stop. Any idea what this is? I’m thinking it’s something to do with this manual valve body. Does anyone know if my old 904 valve body will work in the 727?? Thanks