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I put a np435 from a 351 to a 400. The 351m does have a 11" clutch.
I used to work at a diesel electric shop when I was younger, we built diesel generators. I was constantly dropping off/ picking up flywheels at the local machine shop, and we could make almost any generator/engine combo.
I don't see why you couldn't take a 351m flywheel and a 400 clutch to your local machine shop, and pick it up later that day for cheap, with new holes and re surfaced.
My clutch was still pretty good, but when it comes time, bigger is better