The bellhousings are a different pattern on 351M vs 351C, as the 351M/400 have a taller deck height.
Not a huge deal there, though, as the FMX has a removable bellhousing anyway.
You can use a small-block ford bellhousing (289/302/351W) which IS the same as the 351C.
I'm not sure if anyone makes a stall convertor to fit FMX or not, but you'll want one of those if it's available. 351C's have big valves and ports and are real dogs until you get the RPM's up to around 2500.
You should be fine with the 351M motor mounts, I think. If not then 302/351W ones may work. I used 302 ones on my 351C in a 68 Torino. I had to move the transmission slightly (less than 1") forward, and the crossmember / frame was drilled for that.
Headers... I'd guess that the 72 Torino/Ranchero ones for a 351C would fit your (75 to 79?) Thunderchicken.
Gears ... That's a fairly big and heavy car with a short-stroke engine, so
you'll likely want lower (numerically higher ratio) say 3.50:1 or 3.73:1 with 15" tires for good street performance.
On a big boat like that, you might even want to consider a 460. It WOULD bolt up to the 351M bellhousing, and has tons of torque. The downside, of course, is that they're not known for fuel economy