One of the problems with a replacement head for the GMC is that the bore pitch varies along the length of the block (like the 235 and many other older L6 engines - even the slant 6). Nothing with that chamber/cylinder location pattern exists TIKO.
Clever compromise is possible by averaging out the bore centers and adapting something close (2 X 3 cylinders o a V8 welded, etc.), but not always easy to prevent the chambers from overlapping the deck (but not always fatal), water circ, bolt locations.
The 12 port heads that I've seen (pictures) have really bad chamber designs, some with no chamber in the head (piston dome only), vertical valves, no quench. These were made obsolete in 1949 when GM introduced the 303 Oldsmobile and 331 Cadillac OHV engines.
