I am guessing that the main difference with the older drums is the smaller center/dust cap. Still .... change those suckers to discs if you really expect to share the road with modern cars. It will be one of the best things you do to that car.
I never realized front drums were such a problem. Smaller centers would be a quick fix for a machine shop (lathe).
The later drums would have bigger centers, earlier are smaller, and that can lead to fitment problems trying to combine some early wheels and late drums/discs. Late wheels and earlier parts with the smaller center aren't really a problem. It converts everything to "wheel-centric", using the lugs to center the wheel, instead of being "hub-centric", which uses the ridge at the grease cap and matches the wheel to it. Both set-up are used by different manufacturers, and custom wheel makers. I have early wheels and Granada discs and it is no issue, because Magnums are wheel-centric, but if they were the plain steel wheels from Ford, they wouldn't fit because the centers would be too tight.