whats kind of heads will interchange with my 351m

well ive delt with many 351c and a few m and it sounds like u got a good combination in mind but good luck on finding the closed chamber heads in 2bbl form only the aussie heads were 2bbl closed chanmer and ur heads arent that bad if u do some porting work on them thell flow almost the same as the c heads so unless u just want c heads id use ur heads cleaned up oh and the only diff between the 351m and 400 is the pistons and crank,the rods are the same

That would be money well spent mustangrfast. With just the 2bbl carb on my dirt car, it was a horse off the turns, pulling like a mother. I did the same as you mention on the 351m heads, and they made a ton of difference. You won't have to mess with the c's and I believe you'll be happy with the results. Mine were plain stock, but I did go to the 2 coil valve springs. I had the heads done bare and assembled them myself, pretty inexpensive not considering my labor (I took my time and fired myself after it was done). The rv cam is a good choice also, very good for torque. Let us know how it turns out.