I havent noticed much of a difference in stability between the 03/04 and the 05-07. Both bikes have WER steering dampers, which helps the stability some. They seem to turn about the same. The older 03/04 chassis just feels a little bigger, although its not a hindrance at all as far as handling and rideability goes. Im not sure if the 500 engine would fit the 05 up frame. Someone on here put a 05 up KX250 engine in a 03/04 KX250 frame and it was a pretty easy conversion. The 05 up KX250 engine is completely different from the 04 and below. So, I would think the 500 would go into the newer frame.
I have both the IMS and Clarke tanks for the 04 KX250. Due to the height of the 500 cylinder and the amount the tank drops, there is just no way, with out considerable reshaping and maybe cutting and modifying, that either tank will fit. Ill try to snap a pic this weekend.
As far as the aluminum frame goes, I dont have the fabricating skills required to mod the al frame. I also had the 250 already. I also prefer the steel frame. After riding a few aluminum frame CRs and YZs, I found that even with the suspension revalved for woods riding, there seems to be a noticeable amount of harshness transferred to the rider with the aluminum frame bike over the steel frame bikes. The al frames are pretty rigid. The steel frame has a little bit of flex and has a little bit more of a comfortable ride over the course of a 6 hour, rocky enduro. If I had the money though, there would be a KX500AFX sitting in the garage!
Why are the pro desert riders riding KX450s and KLXs, because that is all thats available to them. Im sure if Kawasaki made a steel perimeter framed 500, they would be on them instead.