Read the link again I posted in your first thread, i.e. this one
The main problem with the 351Ms were crappy stock heads, a horribly low compression ratio, and a ****ty cam. You can use some of the different heads from the 351C to get rid of the first two problems. Your best bet will probably be to find a set of 351C 2V heads (70-74) and then use aftermarket pistons to get your CR to a decent number. Unless of course you wanna put a blower on it. Then just get the 2V heads.
Basically, read the link. It has more information than you probably want.