As you've got a '79 R65, you could probably swap out your 3.44 (31/9) or 3.56 (32/9) final drive with that of an R80/7, or R80RT from the early 80s, which I think came standard with a 3.37 or 3.3 final drive ratio. Of course, you'd need to swap the speedo to get accurate speed and odometer readings, too. Final drive swapping is the "quick and easy" (and much cheaper) way to change the gearing on these bikes. I don't think that I'd go any 'taller' on the final drive ratio than this, though. The R100 of the period that had taller ratios (3.2:1 or 3:1) had much more torque and could pull that gearing effectively, which your R65 won't, especially if 2-up.