Yep, it looks like a 351M/400 alright ... and it could be either as the cylinder blocks are identical for both engines. You *might* get lucky and find a metal tag under the coil bracket.
As has already been said, these engines can be "hot-rodded" ... but IMO, the 460 would be a better platform.
Judging by the paint job and the silicone ... I'd say that someone has recently done some work on that engine. But whether it's been freshened, patched, rebuilt or remanufactured ... it's hard to say.