Shifter- The next problem to be overcome is adapting the shifter. Because the motor shift shaft is shorter on the 500 than the 400 it is necessary to use shift linkage that has a dogleg (see below). In addition, because the 400 lever has a short pivot pin arm and long lever, the end of the lever travels about 6" when shifting. To correct this problem a 750 lever can be used. It is shorter than the 400 lever with a longer pivot pin arm. The adjuster bolt (see below) from the 400 linkage is shorter and must be used or the 750 bolt must be cut off.
The mount width of the 750 lever is narrower (~6mm) than the 400 and a shim washer or grommet should be used as a spacer between the lever and the footrest. Footrest position must also be changed unless you have dinky feet.
Cables- The lower portion of the 400 throttle cable must be replaced with that of a 500. Tach cable must be replaced. All other cables from the 400 will work.
Petcock- The petcock will hit the top of the carb without modification. By turning the petcock about 15 deg and raising the rear of the tank about 1/4" I gained just enough clearance to work. I added a thickness of rubber over the rear tank mount. It may be that the threaded stem on the tank could be bent at an angle to achieve clearance. I didn't try it.
Sprockets- I used a 13/41 tooth setup to stay near the stock 500 ratio.