I think you have to compare what you are changing. The RK use a shorter motor. The Premier is taller. But you need to compare by model. If your not swapping motors too, then you need to compare the gear on the motor to see if they are the same, otherwise the motor may not drive the axle gear.
Where will you get the trucks? MTH requires a turn in for whole trucks.
It may be easier to see if one of the double rollers will fit your RK truck. Lot cheaper fix. G