Welcome to CHR!

The intake bolt angle for the Vortec (1996 and onward) is drilled at 72 degrees versus 90 degrees of 1995 and earlier heads. Vortec heads are tapped at 5/16 – 20 thread. Earlier heads use 3/8 -20 threads.

You can drill and tap the cylinder head for the early style intakes - but you must be very careful and if you make a mistake, the heads may be unusable. Gaskets don’t line up real well and a major danger is drilling to deep on the outer bolt holes and penetrating the water passage. Better bet is one of these GM part numbers:
Part # 14097499 - stock cast iron Vortec intake
Part # 12496820 - low rise dual plane with EGR provision and dual (spread/square) bore bolt pattern.
Part # 12366573 - high rise dual plane manifold with a square bore.

Or one of these Edelbrock Vortec manifolds:
Part # 7116 Performer RPM manifold
Part # 7516 Performer RPM Air Gap manifold

Others may chime in here.. Again, welcome to the site. Hope we can help you in your hot rod journey and that your time here is long and fun too!

Regards,
Glenn