As for the TA in the 300, if it didn't come with one I don't know if the casting has the provisions for the TA. The easiest way would be to swap out the rearend from a 300 or 350 with TA I would think.
70 horsepower from an M is possible without too much work. First, I would bore the block out to accept the 450 sleeves (larger bore), then bolt on a 400 or 450 LP head and you should be just about there. The 450 had 52 horses stock with 281 cubic inches, the LP head should help quite a bit. You can also turn up the RPM a little bit. For better info on the horsepower, post on the pulling forum, they know all about max HP out of an M.