I believe the 37 Chevy(very first 216 motor) did not even have a 'seal', just the 'slinger' to help oil from going out the back. You need to get the '37 manual and remember this was a unique 216, rear seal came later.

Also you may want to contact Beck's Machine (see their ad at inliners.org) these people are among the very few that specialize in these OLD motors.
Best of luck

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed