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Pretty sure they're different. I know the 300 shares the same bolt pattern as the small block V8's (289,302, 351W). If it's a manual, just find a bellhousing. I'm not sure if the 351M/400 pattern matches anything else or not. Still RELATIVELY new to Fords. One good thing about the Chebbies. Same pattern for big-block, small block, inline six, 90Degree V6 and older 4 cylinders. If they'd made it match the Buick,Olds,Pontiac pattern things would be TOO simple on the GM side of the fence.
IIRC the 351M and the 400M have the same pattern as the 385 series (429/460).
The FE series is all by it's lonesome (332-428).
The 351C has the same pattern as the 300 and small blocks - 6 bolt 289 thru 351W.
If your application is a manual then search out a manual bellhousing. It shouldn't be too hard to find. If you are trying to put a C6 351M trans behind the 300 it's not worth it.