Drilling a Magnum intake for '73 'J' heads?

Which M1 intake are you looking at? Mopar made more M1 manifolds than you can shake a stick at- they applied the name to all their manifolds for a while.
Anyway, there probably isn't a snowball's chance in he// I'd bother trying to butcher up a manifold that
A.) isn't the right intake pattern.
B.) isn't right for the carb I want to run.
You may as well cut up a Chevy 2 bbl. manifold to fit it- about the same amount of work.
How about just getting the correct manifold?
Like this M1, among others:
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Or this Performer:
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Or even a stock manifold:
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There's tons of options out there that require no hackery whatsoever.