Man I don't want to pee on your parade any more than what's already been done, but the other posters are correct.
The cheapest and by FAR the easiest is to add the spacers. Downside, the project you put so much effort into won't look good.
If you simply cut and re-thread the steerer you've only solved about 1/3 of the problem, the geometry of the fork legs and springer mechanism will be TOTALLY whacked, unusable.
You can also buy the correct springer fork for your application off-the-shelf, the downside is the darned thing costs 3 times as much and it's essentially the same fork with a shorter steerer and the correct geometry on the other pieces. (This is HUGELY frustrating)
Or, you can do as Scrumblero suggested and fix the geometry of the springer with careful cutting and drilling of the entire fork. This would be my recommendation also. I've done it myself and even though I devoted a solid two hours to it, testing the measurements, and re-testing and re-measuring, it worked out VERY well.
One simple improvement to these springer forks would instantly improve the compatibility of the forks to a MUCH wider range of applications. It's that thick chromed top bracket where the springer bolt mounts to that messes up the whole works because of its fixed angle. A simple but sturdy hinging mechanism on it would allow the thing to work out-of-the-box on so many more projects.
