I machined a piece out of solid aluminum, and I did use longer studs in the front three, they came from a 86-88 KX 500 cylinder. The newer bikes, the 06-08 450 or the 04-05 250f chassis will allow for the use of the rear head stay. I also thought about using stainless steel for the front mount, I bought the material but never got around to it.
The first one I built out of the 99-02 chassis I had to soften the tank to allow for the spark plug clearance and it took quite a bit, the second one I used a newer 250 2t plug which has a shorter ceramic and I didn't have to move it as much.
Good luck