A-body 8-3/4 drums

Agree with all of the above.
Your '68, which you bought in '70, probably just had some new drums thrown on it by the dealer.
And they WILL fit 7 1/4 rears if it was ordered and equipped with 10" brakes. Of course they won't fit 9" brake setups. It's a matter of the size braking system used, not the rear end size.