First, I think this is a wicked cool "ecomod" idea! Keeping the old steel and plastice and making a running gas sipper out of 'em sounds great. Subscribed. Very interested.
I have never restored a car. But confronted with your plablem, I would slow down, don't force or cut anything, research, and ask people. What your doing. You mention RockAuto. What about CARiD? I went over the and entered 1973 Honda Civic and got a hit for brake drums with some specs:
https://www.carid.com/1973-honda-civ...412476769.html
EDIT: Also, HondaPartsNow seems to have 1973-1979 Civic parts schematics and such online, which might help you... maybe they have some of the parts, too... information: