The problem you describe could be misalignment of the input shaft. Are you sure it has a pilot bearing. If the pilot bearing is ok, check the clutch housing. Attach a dial indicator to the flywheel and turn it to check for runout on the clutch housing. If it has excessive runout, sometimes you can shim it against the block. The transmission should not be exerting pressure to try to jump out of gear. Clutch housings can get warped. You never know where your clutch housing has been the last 70 years. Someone may have bought one from a salvage yard that came from a burned vehicle. What prompted the rebuild in the first place? Was it doing as it is now, or did you just want everything new?