I decided to read the documentation that came with the 4412 Holley
Holley 20-95 Throttle Cable Bracket
Holley 20-89 Return Springs
From the 60-66 GMC site
Mr. Gasket Adapter #720-1937.
XRP 700106 Size 6 to 7/8-20 Holley Carburetor Adapter
XRP 239006 Size 6 90 Degree Push-On Hose End
I think I may have the manifold machined flat and weld a Holley base plate onto it.