Mine ran great for over 15 years and that was after years of abuse as a car hauler/wheeler in west branch. I left the hood off of it for a year in my back yard and water got in it and rusted it up. That is the only reason that I going to go with a 460. 400 with hooker headers/edelbrock intake and a Holley 650 worked great had tons of power.
i have many spare parts including the couple hundred mile hooker super comps if anyone is still running a 351M..
And yes from what I have read they are a small block (351C) stroked with a big block bell housing.
Here is a good read on the history of them