Here is some updates...
People ask me how thick should they cut their case to.
That will depend on what you are trying to do.
For a standard clutch assembly simply cut the case down until it is 1/4" lower then the water pump face as shown below.

- www.oem-cycle.com kx500 two piece clutch cover 101.JPG (44.54 KiB) Viewed 22474 times
Now, if you want a Rekluse clutch there is two different ways of doing it.
I highly recommend that you get the parts together first then measure to be sure that the measurements will meet your needs.
1. If you use a KX450F clutch cover then you need to machine the clutch cover to the same thickness as the water pump cover face, NOT 1/4" lower.
2. If you use use a KXF450 (Quad) clutch cover, then you can machine the clutch cover 1/4" lower then the face of the water pump cover as shown in the pic above.
Again, measure first... then cut
Here's another tip.
While you have the clutch cover in the milling machine, remove the web as shown in the pic below.

- www.oem-cycle.com kx500 two piece clutch cover 102.JPG (67.21 KiB) Viewed 22474 times
If bikes are for kids I'll never grow up.