I posted some similar questions a while back and had no responses. I have an M70 engine project that I am working on and was searching for gear box options and performance parts for the V12. I ended up making a mountig plate for the back of the engine to bolt up a Muncie 4 speed, I also have a T5 Mustang 5 speed that will fit, but w/a small amount of machining on the front snout.
I thought the stock intake runners were on the puney side, so I fabbed up my own intake manifold to fit 3 Weber down draft carbs. I adapted dual 6cylinder Chevy distributors to drive off the front of the cams and I trashed the crappy exhaust manifolds in favor of fabbing up a set of custom headers. At least w/carbs and basic distributors, I won't have to modify an ecm system and have electronics to deal with. Besides, it'll have less clutter under the hood. I have several '60s muscle cars, I just need to decide which car to eventually put it in.
I found a guy in Canada that punched one out to a 6.0 and has a supplier (or he makes them himself) for camshafts. I'll have to pull up his file, but I think he quoted a cam and spring set at $900.
I always wondered what an M70 w/open pipes sounded like, check out the vids. This is one bad home-built.
http://www.ultimav12.ca