Enginerd, whats up? I can remember that you wanted to swap the 46mm fork of your ´89 KX250 into the KX500... thats very easy and made in only one hour, without removing the front wheels. bearings are same size, same brake frontwheel and so on... 
You have a good memory! I do have a complete 1989 KX250 front end with the 46mm KYB forks. This will be a direct swap, like you said. I like this idea because it would be so simple. However, I also have a complete 1996 RM125 front end with 49mm Showa forks as well. These would probably be better forks, but not as easy a swap. I just don't know which way to go. I've also been talking with Sandblaster about adapting a late model KX250F front end into the old K5. I have too many choices! 
my memories are legendary...
which fork is actually in your kx500? i think it is the 41mm usd-kayaba.
my opinion is first always start with the easiest things... you should start with the 89 46mm fork swap to the k5 and maybe you need harder springs because of the additional engine weight of the 500 against the 250...
test the performance on a ride and then you have the option to swap the fork back in the 250 and try the next thing.
i cant wait to here how it performs...
and i also shuffle my feet till the time comes for my fork project, but health comes first, so i have to wait and ride my playstation sx-races...
