833 reverse pops out (1st post on board)

My a-833 overdrive does not want to stay in reverse. I rebuilt it. Replaced all synchros and bearings and re-assemble it by the service book. Before I put it in the car I went through all the gears to check for shifting/meshing problems and everything was ok. It went trough all 4 gears fine and reverse droped and locked right also. The problem is when I put the tourqe of that 360 to it it wants to pop out of reverse. I have adjusted the linkage out to its limits but still no help. It is a 76 alluminum cased overdrive and I have the 64 ball and detent sideplate on it. Could it be something to do with the sideplate not allowing the gear to fully engage or the ball and detent on the reverse lever worn. If anyones had this problem before let me know becuase I'm tired of looking for pull through parking spaces. Thanks in advance.