I plumb forgot about the fact that the Passports use the same primary gearing as the majority of the 50's. Using the primary gears from an XR/CRF-70 would lower your cruising RPM's on the road substantially. I don't have the numbers here in front of me but the difference was substantial enough that changing to the 50 primary gears was a hot rod trick I did to the 70 motors to give them more accelleration. By contrast using the taller 70 primary gears would give you a more relaxed engine feel on the road.
The changeover requires the front and rear primary gears from the 70 plus the gear that goes inside of the clutch and takes maybe a half-hour to complete.