My '37 has the original toploader 3sp.
It has bosses on either side of the transmission as mounting points for (left side) brake master cylinder and (right side) emergency brake.
The rear of every Chevy '6' block (216, 235, or 261) is the same from 1937 through 1962, and the flywheel/crank-flange pattern is the same from '41-'62. So theoretically a 3sp from a '40 would fit the '37.
Here's a pic of the original configuration on a '37:
http://i1098.photobucket.com/albums/g369/J-reb/1937_Chev_Pickup/37_Chev_Trans.jpg
