If I put 351cleveland 2v heads on a 400 mmc would that not increase the compression ratio quite a bit? the 400 mmc has .50" more stroke with the same bore. What do you think?

The "M" series engines use the same head as 351C 2-bbl. The chamber volume is the same. The piston in the 400 will have a "dish", where the one for 351s will be "flat".

Jim

If you want some power I would suggest the 351 C H/O or the CJ heads.