The 351M is part of the same 335 engine family as the 351C and 400 smallblocks. It is basically a destroked 400. The 400 is a Cleveland with taller deck heights, and replaced the old 390 FE in pickups because the FE couldn't meet emissions requirements in the 70s. Demand for a 5.7/5.8 liter engine remained strong, but the Windsor plant was already making all the 351s it could. With the 360 FE going out of production as well as the 351C, Ford dropped a crank with 1/2" shorter stroke into the tall 400 block to make the 351M so Ford could offer pickup & Bronco buyers roughly the same engine choices as Chevy & Dodge. It either has really long rods or tall pistons to make up the difference.
It's a 70's era smog motor that was never designed as a performance engine. But you can wake them up. Edelbrock and Cylinder Head Innovations (CHI) make performance heads that will bolt onto any 335 engine. Intake manifolds are few and far between. Edelbrock makes a performer dual plane intake, and Weiand makes an action plus dual plane. CHI makes a single plane. That's about it. Your situation with cams is better, as any cam that will work in a cleveland will work in the 351M, and any distributor that will fit a 335/385 series engine will work as well. 351C headers will bolt onto the engine, but the engine is wider due to the taller deck height so cleveland headers may not fit the application.
The good news is that a 400 crank will drop right in. The bad news is that this isn't going to be a cheap build up. Is it worth it? That's up to you.