Why is my slanty chugging? (Problem solved, for now anyway)

15k miles with leaded gasoline, 30k with unleaded was the standard in the '70s. We have better spark plugs now, and cleaner-burning gasoline, so you should be able to go a long time on a set of plugs. If putting in new plugs stopped the misfire, it reinforces that upgrading your ignition system (as a whole) will likely knock down the car's seeming need for brand-new plugs to run without misfire.