matts67coupe

I am also putting a 351 M in my mustang a 67'. You can make your 351m into a 400 by changing the crankshaft and getting different pistons. Get flat top pistons from a 72 or other early 400 engines. They make high performance parts for them you just have to look. I have about 3500.00 worth of hp stuff in mine. You can also get 4 bbl heads also which i have and that helps a lot with hp. Jegs and summit sell hp intake manifolds for the engine. They arent ideal but like you I have one and it has a lot of money in it. It has around 9.8 compression and should put out about 300-350 horsepower.

rmodel65

www.ausfordparts.com has parts

andrewmp6

The downsides are harder to efi and no overdrive tranny with out a adapter plate.

tarafied1

This months Mustang And Fords magazine has an article on the 400M. They got nearly 600hp with basicaly stock short block. It had Cleveland flat top pistons (Probe). Aluminum heads and they tested several cams, intakes and carbs. It made gobs of torque, in fact over 400 ftlbs around 2,000 rpms and above!
The 385 series (429/460) motor mounts and Cleveland headers will work. The cams for any Cleveland will work and there are spacers that allow the use of any Cleveland intake.
I have a 429 in my 67, trans options are limited but they do make adapters for just about any auto trans and a Lakewood bell housing would allow for just about any manual trans.
While I agree there are other options, if you have an M already I'd say go for it.

my77stang

I'd say go for it (as long as your gonna turn it into a 400).