drilling magnum heads

You would not use the intake as a drill guide. You can use it to locate and mark the LA holes, then you would drill the holes using a drill press or very careful hand drilling.
The holes in the dual pattern intake are too large to use as a drill guide.
I am considering doing this myself but I just laid the LA gasket over the intake and coolant ports, center marked each LA bolt hole with a fine tip marker, and then laid the intake in place with gaskets and verified the holes line up with my marks. I have not actually drilled it yet.

Edited to add:
The jig shown in post 2 is the best idea for doing multiple sets of heads. But for just one set, mark it and drill it carefully.