So here is what we ended up with so far.
KX450F rear wheel
KX450F to KX500 rear axle adapters from Warp 9.
A used 94 CRB600 f2 Right front brake caliper.
KX500 01 rear brake caliper mount.
KX500 93 rear brake hose.
Nothing has been rebuilt.
All I did was disassemble the caliper, clean it, and reassemble it.
Modded the mount to make correct clearance and removed some unnecessary material, then bled and adjusted it.
That's it.
To do this right I should completely rebuild the master cylinder, the caliper, installed new pads (I used the pads that came on the used caliper), installed a new SS brake line, and installed a new rotor.
However, even with all this old used junk the performance is remarkably better..
But how much of the increased braking is from the larger rotor diameter and how much is from the dual piston caliper?
We will have to test it and find out...
I believe this is the final version for the CBR600 caliper..