There's just about zero benefit to using spacers on a threaded headset. Obviously the threads have to extend beyond the headset so the threaded upper cup can fit. That means you need about 1" of thread below the top nut of a typical headset. But there's a second requirement, and that's that the stem extend into the fork beyond the threaded section of the fork because the depth of thread is roughly half the wall, and the steerer needs the stem to carry stresses below this weakened area. How far beyond the thread is necessary is hard to say, but figure 1/2" at a minimum and up to an inch desirable (more than that won't help).

So if you measure the fork very carefully and have the thread end just below the upper race, you might be able to add 1/2" of spacers under the locknut, but that's about it. If you need more height than a typical quill stem affords, you have a few choices.

1- use a stem with an obtuse angle so the forward extension rises gaining you more height.
2- use one of those "tall boy" stems - though, IMO, they tend to be pretty crappy.
3- if you're buying a new fork anyway, go threadless which will afford some more choices, and the ability to add up to 2" of spacers, but don't forget that this won't net you much because you lose the 2-3" of height that a quill stem has above the headset. One nice thing about the threadless option, is that there's a nicer selection of quality rising stems (any mounted with the smaller angle on top.

Before spending any dough, take the time to figure what you're trying to achieve (desired handlebar height) and use this stem calculator to see how the various options play out.