Jack the wheels are both 16''x 3'' I imagine but I believe you will find there is a lot more to it than just the same size wheels.
Assuming the wheels are the same size with the correct size axles...
The RK wheels will be for dual front disc where the FB wheels will not.You would have to install sliders to accept dual front calipers which would require a larger master cylinder and different brake lines.
I could be off here but there's a few differences off the top of my head.Maybe somebody will come along that has actually done this swap.
If all the above is involved you will probably find it cheaper just to buy new wheels.
Yes all you may need are diff spacers. for a couple bucks at your local harley shop. The other guy is right about the duel disk but all you have to do is unbolt the right side disk and your ready to go!%potstir:chopper: