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

Do not modify that intake manifold. It's worth far more than its LA equivalent, probably close to double. eBay it and find a Strip Dominator or Victor.

That being said, the Magnum holes are in virtually the same place as the LA holes. They're just at a different angle. The two different angles intersect at the head. Look at a Speedmaster intake to see what I mean. Could you do it at home? Probably. Should you attempt it? Probaby not, especially considering the value of the intake.