Anthony, do I dare ask WHICH vendor this was...the ones that "were not 'licensed mechanics'"? I've known an awful lots of vendors and parts counter people who may not have been mechanics but DID know their parts at least. Geesh!
If the shoes are 8" x 1.5", then they might be same as or similar to the very late (1973) GT6, which in turn shared brake parts with (I think) the Austin/Morris Marina.
Ultimately you might try to find some really good vendor who happens to have a comprehensive Girling catalogue that would show pictures or drawings of various brake shoes and have cross-reference information. Years ago I was able to easily identify and then obtain brake shoes for a Triumph Mayflower, thanks to a very helpful local parts store and their catalogue.
I see you're here in Albany, NY, correct? You might try some of the locals here such as Anca's or Foreign Autopart or Bap/Geon...or Coveland Motorsports in Schuylerville or Kim's in Amsterdam.