351M: 335 series engine. Very heavy! Big Block bell housing bolt pattern, 2 peice rear seal, No Coolant flow through intake manifold, water neck on block, unique motor mounts, shares same head bolt pattern as windsor series, but heads will not interchange without tons of work & custom intake, little aftermarket support compared to Windsor or 385 series big block. Can be stroked to a 427 and made to run very well.
351W: Tall deck 302. Small Block bell housing, one peice rear seal (1984+), roller cam OE (Late 1995-96), Coolant flow through intake, water neck off intake, conventional motor mount, unique oil pan & intake, tons of aftermarket performance & parts.
If it were me and I had a 351, I would look for a 1996 351W to rebuild and drop in a 392 stroker kit. Add a set of gt40P heads and a mild comp cams roller, and you will have 325-350 reliable hp that idles like a stocker.