As far as 351's go, I have had a bunch of 351C, a couple 351W, and almost no 351M (and 400). The 351C is an absolute torque beast in any configuration, the 351W has great potential, and I love the SBF familiarity factor. The 351M and 400 seemed like any other late 70's engine that had been nerfed for fuel economy and emissions. Those last two ALWAYS had open chamber heads, but it was the same design as the 351C, for which there were multiple versions of closed-chamber heads made. A closed chamber swap on a 351M or 400 would be an instant compression boost, as well as getting you much larger valves and ports.