here's a link for Muncie Identification, one of the best in my opinion.
Muncie 4 Speed Transmissions... so far seems to me you have a 26 spline d M-20. made in 1970 to 74. But you still need to pull the side cover to check the (cut) of the gears to know for sure. Also to V8-4spd... not all Muncie trans case ear were broken from torque abuse. A really common failure is using the bellhousing to transmission bolts to line up with the pilot bushing. Also improper grade of bolts. I used Grade 8 bolts, flats, and lock washers. But I always torgue them properly, I replace my lock washers at any teardown too. Also a transmission jack is worth it price ( in broken case ears
) in proper placing of your transmission.... as opposed to the wood block on a floor jack as we all have done