The advantages of OHC were not readily understood by the hot rod public 70 years ago. It does free up some power from the drive, and reduces component weight and compliance (you can't buy good GMC rockers last time I looked), but that's the whole list.
The GMC head won't produce power much above an RPM limit easily reached with pushrods.
In theory, OHC allow the valves to be in better positions w/r/t their ports (and no pushrod holes!) but SOHC requires rocker arms.
The staggered bore pitch (like the 235 but larger) makes adapting a better head even more of a headache than the Falconer: bolt, water locations, chamber overlapping the head deck (like the 235 does now).
If I wanted a serious (but not legal in XXO) GMC I would be looking at a big inch 4 valve DOHC engine. The GMC average bore pitch looks like 4.45" (3 @ 4-3/8", 2 @ 4-9/16") or 113mm. The Nissan V6 (including GTR) is 112mm. Centering the #3 & 4, the error adds 1mm for each cylinder in both directions.
The TB48 L6 engine may also be close with the advantage of a single casting but the bore is huge at 3.917".
The Toyota DOHC L6 FZ also looks interesting.
