Are you sure you'll have a compatible fork? Very hard to cut more threads if there's not enough. This happened when I put a 1" suspension fork on my Trek 800. The tube on the new fork was 4 or 5 inches longer, and the thread ended about 2 inches above the frame. I had a die, but it was only for steel plumbing pipe and unable to cut the harder steel in the fork. When I learned a proper bike tool was over $100, I took the advice over in the mechanics forum and converted the fork to threadless. (No, they hate people like me at the local bike shops. No way I'm taking my bike/fork there.)

They also make a guide for cutting the fork thru the threads. I had to cut mine there anyway, and I used a sawzall and the old nuts as a guide. I recall that the nuts spun off afterwards, so if you're careful, you won't necessarily wreak the threads.

Ask over in the mechanics forum too.