Question: what is the difference between a 351 windsor and modified?
Answer: The 351M is part of the same Ford 335 V8 engine series as the 351 Cleveland. It's based off of the 400 Modified engine (see note on nomenclature in opening paragraph), which was essentially a 351C with a longer stroke. Unlike the 351C however, the 400 block had the bellhousing bolt pattern of 385-series big blocks.