I have a R20 frame and a very similar Kama frame (made in USSR, converted to road single speed bike), and obviously I decided for 451 rims because in first place it is easier to find tyres for this size whose width is arround 25-28 mm and pressures 100 - 120 psi. Second The R20 frame is adopted to use with 451 mm wheels and not the 406 mm wheels, giving you advantages on adopting modern brake technology to the frame - rim and also bigger wheels set your bottom braket higher obove the ground giving you more pedal clearance.