A few years ago I lost a bet arguing this point, so I have some experience in what I'm about to say: The difference in the ucas is that the 73 and later setup used a large ball joint, whereas the 72 and down A body uses a small upper ball joint. It has always been assumed that drum brake equipped cars built from 73-76 used the small ball joint UCAS but many do not! It is quite possible to find large ball joint UCAs on a 73-76 drum brake car. In fact, since I lost that bet with an FABO member, I've never seen a 73-76 UCA That was NOT big ball joint!
I can't confirm that they don't exist. I do part out about 10 A bodies per year and I haven't found any yet, so the odds are good that you've got the correct UCAs even if your 73 and 76 have drum brakes.