The clamps do not interchange due to the Regular forks have a smaller Dia then the 750's USD forks. If your stem is the problem they could probably be swaped as the 600 and 750 use the same bearings.
For the rear wheel get with Fastcat as she has done the SRAD rear wheel swap. I am changing Swingarm on mine and I'm still waiting on it to come back from the welders.
Yeah the thread on the stem is stripped for the top clamp. I wonder what the stem itself would cost? probably more than the $50 the guy wants for the 600 stem?? I'm swapping to a '92 1100 arm on this '92 750, i had a 6.25" RC comp in there on the arm before so the srad wheel should be no real problem to fit (hopefully) I plan on making all new brake rods anyway, plus modifying the arm for drag racing so its all pretty custom!!!!