[gmc684x4]

Rwd trucks generally used rams horns that angle dump to help clear the cross member
4x4 trucks have the engine back and use straight dump rams horns

I am going to say with out a doubt you did not thread chase out the bolt holes in the one head as they old heads and vortec heads use the same pattern

The other thing to check after you thread chase the bolt holes is to see if your manifold has warped in any fashion to distort it too much to bolt in
Check face with a straight edge to see if it is flat and not cupping the outer bolt holes throwing them off alignment