Have you seen this thread, post #37
https://www.67-72chevytrucks.com/vbo...=790866&page=2
Where Todd mentions the wider frame rails of the earlier trucks?
Seems like you have this worked out. I would bet, if the factory engine stands are interchangeable between C10 and C20, the Holley probably would be too.
I was always under the impression that the C20 frame rails are taller, but that may not start until under the cab.