[George/Maine]

I think a 51 mercury bell housing is a half bell. They had a 6 even spaced flywheel bolt pattern .What trans are you using one with 1" spline or larger 1 1/4"
If mercury flywheel throw-out bearing is larger.
If you have a 1953 car it uses a 9 1/2 clutch.
You should only Ford 49 to 53 parts. If you had the flywheel re drilled could be wrong.
Those new PP are not to good fingers like sliced bread. No Factory Adjustment screws at end of finger. Save money and relive the days gone by and don't fix.