I think I may have the manifold machined flat and weld a Holley base plate onto it.[/QUOTE]
It wouldn't be necessary to go through all that, what I have done is opened up base on the manifold with a die grinder to match the throttle plate bores of the Holley as well as open the bores of the adapter. You don't want the carb sitting too low on the manifold. Many GMC V6 engine used spacers between the carb & manifold, this helped with velocity. Some 351C and many 379 engines in Medium duty trucks actually came with a 8 to 10 inch tunnel ram