904 Tranny not shifting in drive

A buddy of mine and I built a 904 tranny for my dart. It will manual shift thru the gears fine but when put in drive it goes into third gear and stays in third. It won't even downshift out of it. Have tried two valve bodies in it and does the same thing. Anyone have any ideas what it could be? My buddy thinks there may be a crack in the front piston assembly.
Thanks for reading.