Hi Vince,
Original engine was 400 Clev +C6, but something strange there were with those rotten backplates, as they seemed to be a bit deeper than what normal 2" brake shoes would need. Brake shoes were wider than 2", but not as wide as 2.5" - difference is/was visible.
So, I assume that somebody had replaced the backplates already earlier - with something "close to match" and with a bit wider brake shoes, as these will fit to the larger size finned drum anyway.
So, to get everything back as really should be, I hunted these correct deeper backplates in the first place, and yes, I did find a very decent pair.
I have another wagon & rear axle, that is from an 1972, 351 Clev + C4 car and those brakes are original and surely 10x2". These have the larger size finned drums also.