So were the bolt holes on your old clutch with the 11 inch disk have the holes right on the edge of the flywheel? The larger 9 bolt heavy duty clutch pressure plate had a larger bolt circle than the standard 6-bolt clutch (10 or 11-inch vs the standard 9-1/2 inch disk.) So it sounds like your new clutch must be some other iteration than the two that Chevy used back in the day.
Is your flywheel flat all the way across (except for the center bolting area)? A flywheel with a raised rim where the pressure plate bolts on would explain why a pressure plate set up for a flat flywheel didn't work correctly, as the clamping force would be less with the disk recessed relative to the pressure plate mounting surface.
