R1 tires set to maximum tread width are wider than r4 tires making the machine more stable on hills. Not less stable.If you have experience I your terrain with that type of tire I see no reason to go differently. A few things about R1s...first they arent as thick of a wall so they will puncture easily (not sure about your 1979 versions but todays probably arent as thick). You might want to consider some R1s with more plys if you can get them. Second, R1s will raise the center of gravity of the machine a bit making it more prone to roll. If you have sidehills this isnt always a good thing. Consider dealer installed wheel spacers or flipping the rims if available on that machine to widen it.
Other than that, great machine and best of luck!