MP valve covers knock out machined for pcv grommet (Chevy style)
I believe this is what would have worked for you if the breather (or whatever) requires a 3/4" hole. I have personally confirmed this on my own MP cast valve covers - perfect fit (replacing the MP PCV grommet):
Ford Performance Parts Valve Cover Grommet Sets M-6892-F
My photo below. MP and Ford grommets are dimensionally the same except for center hole:
Nice machine work, though!