I have a Question can i simply put a 351m crankshaft int a 351w engine block or would I have to make modifications to it. I see that the 351m and 351w crank have the same mains, stroke and rods. If i could put the 351m crank into 351w what would i have then?
The 351M shares the main journal spacing of the 351W and the journal sizes as you stated. It will bolt right in to a bare block, but I believe there is a difference in the snout which means it either has to be modified (machined) or you would have to use the 351M balancer etc. and the pulley might not end up in the wrong location fore and aft.
Before the advent of readily available stroker crankshafts, 400 cranks used to be modified to fit the 351W and the front snout was remachined to suit the 351W balancer.
The counterweights probably are larger on the M engine than on the W engine since the con rods and pistons are heavier on the M engine. If this is so, then while the M crank could be made to work, it will require a lot of machine work.