is that height with the spring plate removed and 2 steels and one frition plate inserted in place of it? If that is so, you could put it back together and have a go without the spring plate, less stack height will mean less pressure from the clutch spring and possible slip, it will go back together OK.
BUT you need to swap the spring plate for all three - the spring plate in the OEM clutch is adjacent to 2 friction plates, and so you need to maintain the friction/steel/friction layout. I would have thought it would have been thicker but there u go.
Remember that new friction sets for the clutches for the 4 speed evo models come up on ebay all the time, I got my last aftermarket one for about $40US. like I said they aren't as good as OEM I don't think, but whilst you have it apart it might be worth putting new frictions in too. Have a look at the grooves in the friction material, as they wear they tend to get very shallow (like a tire wearing out) but I'm not sure off the top of my head their stock depth.
You are probably aware of the 2 problems wit the early evo sportsters primary and transmissions - the magnets shedding on the back of the clutch basket, and the shifter cam breaking in the transmission. If you have it apart it might be worth replacing the cam with a steel aftermarket one, and checking the condition of the splines on the clutch shaft and hub as well as the magnets for looseness on the back of the clutch basket.
I have a few pics of this on my website i did a few years ago, should link from this site.