Since my bike is 42 years old and I'm going to be in there anyways for my engine build, I am completely replacing my transmission. Do I need a new trap door?
It is highly unlikely that you would need to replace the trap door. Also, it is likely that it may not line up the main and counter shafts correctly with the case. Best to keep the one that was mated with the case at the factory.
Was there a problem with the transmission that you need to replace it? Are you installing Andrews gears?