Hey, I also have a 79 RS.
I had Avon Roadriders on for a while... 120s... really liked them. Good amount of grip and feedback (for an airhead
)... could take right over to the edge riding 2up and good mileage too (got over 10k on the rear and have now done about 21k on the front!). Incidentally the 120 did rub on the inside of the rear fender but only if I completely bottomed out the suspension. Never had an issue with it rubbing on the swing arm / drive shaft.... but it was a bit of a pain to take off the wheel.
The rear is now a Dunlop D404 on a 110. Its ok, good grip but not as great mileage as I had out of the Avon (done about 8k miles with this one and it is now toast). Mind you I have been using it on worse roads.
Hope this helps.