Here's my two pennies
The head tubes on the Wicked (and regular Fat) are machined from thick-walled 4130 mechanical tube. Without having my own Wicked right in front of me, I believe that the thicker ends of the head tube are about 5 - 6 mm in length (as opposed to the machined center) Since you have a 1990 Wicked, I will assume that it has a chin gusset which is welded to the bottom of the downtube and the head tube and only leaves about 2 - 3 mm of the thicker section showing. You can face the bottom of the head tube a little bit as long as you leave at least 2 mm of the thicker section extending beyond the weld of the gusset. I would not take more than 1 mm off of the head tube here though, this area is highly stressed during hard trail riding especially with a unicrown fork (assuming this is what you have) and will alter the geometry by a small amount (head tube angle will increase)
There are two other areas where you can gain some space. The top of the head tube is subject to far less stress and that can be faced down about 2 mm and this will not change your frame geometry. Both of these can be done with the correct tool, good cutting oil, and the confidence and skill of an experienced bike mechanic.
One other option is to turn down the crown race on the fork. You may be able to gain another 1 - 1.5 mm here but this will change the frame geometry a little bit just as facing the bottom of the head tube will also change the geometry a little bit. Is that enough to notice while you are riding? I can't answer that for you.
In order to turn the crown race on the fork, I would recommend that a machine shop with a decent lathe be employed to do this. Look at your crown race carefully and see if there is a enough material there to turn a millimeter off of it before even contemplating this.
Best of luck and keep us posted
Scott