I'd address anything else that would be difficult to with the engine still in the vehicle. I'd at the very least check the compression and make sure it's good because it's much easier to pull the heads and rotating assembly to clean up the bores and rering the pistons when the engine is out of the truck vs. in the truck. Ditto with listening very carefully for noises like a rod knock or noisy lifters. I'd also at the very least give the engine a good wash and a repaint.
The other thing I am sure will be suggested to do by others when you take your 351M out is to put a healthy 460 in its place. It's a drop-in replacement as they use the same bellhousing pattern and a 460 will be much stouter than any M-block in a similar state of tune. Personally, I'd turn the 351M into a 400M using the 400M's crank, rods, and get yourself some higher compression 400M pistons. (The 351M and 400M use the same block.) I'd also put on a good 4bbl intake and carb, cam (and check the timing) headers, and exhaust. The M blocks were very doggy from the factory due to emissions issues and what I described above will un-dog it quite nicely.