Thanks for looking into this swap Sandblaster! I have a few comments that may help someone who is considering this swap.
The first thing I found when I began researching this swap is that the KX500 and 07 KX250F share the same lower steering head bearing.
This allows the 250F triples to slide into the KX500 frame, and everything is taken care of as far as the
LOWER bearing goes.
The
LOWER bearing (All Balls Racing, Part #99-3519-5) dimensions for both bikes are:
30mm I.D.
55mm O.D.
17mm Height
The
UPPER steering head bearings are different between the two bikes.
The
KX500 UPPER bearing (All Balls Racing, Part #32205-5) dimensions are:
25mm I.D.
52mm O.D.
19mm Height
The
KX250F UPPER bearing (All Balls Racing, Part #99-3513-5) dimensions are:
28mm I.D.
52mm O.D.
16mm Height
As you can see, both the KX250F, and the KX500 use the same O.D. on the upper bearing.
This is good! 
That means that the
UPPER bearing pocket in both frames has the same O.D.
However, the two bikes use a different I.D (25mm vs. 28mm) on the UPPER bearing because the 250F uses a steering stem with a 3mm larger diameter.
The solution is to use the 2007 KX250F
UPPER bearing and race in the
UPPER location of the KX500 steering head.
This should work because the KX 250F
UPPER bearing (All Balls Racing, Part #99-3513-5) has the 28mm I.D. to fit the top of the 250F stem, and a 52mm O.D. to fit the
UPPER bearing pocket on the KX500 frame.
Also, the two bikes use a different bearing height or thickness (19mm vs. 16mm) on the UPPER bearing because the 250F uses a shorter, or thinner
UPPER bearing.
To solve this problem, we install a spacer or large washer into the KX500
UPPER steering head bearing pocket
PRIOR TO installing the KX250F upper bearing. This spacer or washer must have a thickness of 3mm (19mm KX500 bearing - 16mm KX250F bearing = 3mm spacer needed), a 28mm I.D. , and a 52mm O.D.
Following the previously described procedure
SHOULD allow the KX250F triple clamps to be bolted into the KX500 frame without the need to have a new stem pressed into the KX250F triple clamps. Then we can continue to address the issue regarding differences in the steering stop locations between the two bikes, as Sandblaster has so beautifully done in his pictorial posts above.
