I might be wrong, but isn't the M21 a 4 speed? I seem to recall that they were used in big block Camaro/Chevelles with the 396 or COPO 427. Also in Corvettes. If you want a 3 speed, you'll want an SM 318/319 or the later SM 326/330, preferably with the Borg-Warner overdrive. If you want a really tough 3 speed, look for the B-W unit that was an option in the 3/4 and 1 ton models.
