Is Simon really impossibly heavy? Or, bigger question: does Simon have steel wheels? There is a fair bit of variance in the steel tubing used in bikes built during the 70s and 80s, and steel wheels are awful and weigh a ton. I would say you are better off with a bike from the eighties than one from the seventies, just because they tend to be lighter and will almost certainly have aluminum rims. You can replace the old rims but they are probably 27" wheels, a little hard to find without mail ordering, and will probably cost you more than buying a newer garage sale bike with aluminum rims.

We have one '75 Centurion that rides like this. Even after my husband completely replaced the drive train and turned it into a single speed with all new parts, it still is no fun to ride, and that's because it weighs about 30 pounds and just doesn't have a good feel to it. It has steel rims, and the frame itself is much heavier than any of our 80s steel frames.

I'd look for something about ten years newer. That's the easy answer.