I’m looking for a set of original rear leaf spring/shock plate brackets for an A-Body with an 8 ¾ rear axle. I know there are different plates for the various axle sizes. Any quick way to identify the plate? Dimension from the side or bolt hole centers or key features on the part, etc…?

Both of those appear to be correct...I think the driving force on whether they're correct or not is if they have the U bolts spaced correctly for the diameter of the axle tubes.
There was a decent post on them a while back until the usual suspects turned it into a pissing contest...

There was a recent post that came to the conclusion that all shock plates are the same except for one weird year where the E body car used a unique shock plate.

Thank you for the feedback so far. To confirm there are differences between the plates 7.25, 8.25, 8.75, correct?

7 1/4 axle tubes were skinnier, thus a different bolt pattern than 8 1/4" & 8 3/4" shock plates.