The 218mm (8.583") pitch is close enough to the nominal 8-9/16" as being Chevrolet, but the 1.732" ports are bigger than any stovebolt head.
Unfortunately, in that era there was this mistaken belief that engines were literally 'starving' for air and that adding carburetors and opening up intake ports would assist in 'breathing'.
My guess is that the original alignment-ring bosses have been hogged-out/smoothed to match an equally hogged out intake port on the head.
As a result, the center-to-center dimensions are key - not the port diameters.
