I had an '83 that I used as my workhorse fix gear for 30 years and 27,000 miles until I shortened it substantially from a crash I walked away from unscathed. I wasn't a fan of the fit but materials and workmanship were first class.

I wouldn't sweat that rust at all. Run some Framesaver of AMS Heavy Duty Metal Protector inside the tubes and call it good for then next 20 years. My 610 got ridden year 'round in Seattle and Portland. Locked up in the rain hundreds of times.