Resto44, I am in the process of rebuilding my SM420 4 sp in my '50. I went to a local rebuild shop to ask them what it would cost for them to rebuild it. They gave me a price with exchange with the proviso that I must have the matching top plate to the transmission because the shifting forks are set up by the factory to match the transmission. This is probably your problem.
Steve O


1950 Chevy 3100
261, 4 speed
Purchased 1977

1964 Pontiac Lemans Convert