Well.......it can work on the block plate if the amount the starter needs to move out is small and that's often usually the case. There is some 'play' to be had between the stock plate hole and the starter's lower ear.
In other cases where it isn't enough, clearancing that large hole in the plate with a die grinder, where the starter's nose resisters in the block plate (more work, for sure!) provides more room for the starter to move out. It's usually not a lot that needs to be removed from the frame side of the big old hole either. Agree too that one should always use the OEM plate.
With an OEM block and an auto tranny plate this would be a rare modification. But, with an aftermarket blow-proof stick bellhousing that is centered, either fix is sometimes needed to bring sufficient clearance to the ring and Bendix gears, as I found out.