The tranny in my Elkie has a maincase casting number of 3925647.
Appears to be a Saginaw 3 speed (RPO M15).
Here is the Three speed tranny with the old three-on-the-tree linkage
and here is the elkie and cross member with a temporary M22 4-speed in place...
I believe this is the correct cross member to bolt an original Muncie shifter to, but I'd gladly put in a Hurst 3-speed shifter if I can get one for a reasonable price.
I'm pulling out the M22 because I'm going to sell the Elkie and I sure won't get value for the M22 4-speed bolted to the 307 (although it made the Elkie much more fun to drive).