IIRC it can be adapted to the Chevy 250 etc. (not a direct swap since the Pontiac-only ports are different, although I think the port positions match).
I did some searching, and no explanation as to why the huge volume difference between front and rear cylinder group plenums. Not visible but possible: the front leg (shown right, above) may be deeper, the rear leg is shallower for some reason and makes up the same X-section with width? The firing order (on this picture) is #1 (right), #5 (left), #3 (center), #6 (left), #2 (right), #4 (center), is this significant?
Beware: QJ to Holley base adapters notoriously have small gasket surfaces mating to your manifold, and are leak-prone.
