I've listed this info at least once (I think, but may be confused by the number of times I've PM'd it to folks). It is VERY easy to convert an older Zipp freewheel/fixed gear disc (950 or 1150) for track use without the specific track axle. I don't still have the wheel I converted, or I'd post pictures.
Here's the deal:
First, its helpful to envision things before you start. The wheel should be spaced for 130mm or perhaps 126mm with the road axle configuration. You want to respace it to 120mm to work in the typical track frame. So, you're looking at removing 10mm worth of spacer (or 6mm if starting from 126). In the case of the disc that I converted, the spacer on the freewheel side was 5mm shorter than the spacer on the fixed gear side, so it was pretty simple--- instead of taking 5mm off both spacers, I put the shorter spacer on the fixed gear side and cut the required 10mm off the longer spacer and installed it on the other side. Now, the wheel is spaced to 120mm and still centered between dropouts.
The road axle isn't threaded for the Zipp track caps, so you'll have to use a bolt-on skewer. I recommend using chain tensioners, too, to insure you don't pull the wheel out of whack in standing starts or max sprint situations. The road axle is intended for use in a frame spaced to 130mm, with no protrusion beyond the dropouts (otherwise the skewer won't hold the wheel in the frame). So, once you've fixed the spacer issue, install the wheel into your frame to determine just how much axle must be cut off on each side so that it doesn't protrude beyond the frame. Don't forget to factor in for the width of the chain tensioners, too, if you decide to use them.
Keep in mind, if you cut the axle too short, you can't put it back. BUT, if you cut it too long, the skewer will bottom out on the axle end instead of the frame, and you won't have a secure mounting. Also, if you cut it to fit a frame with wide dropouts, then decide to use the wheel in a frame with narrower dropouts, the axle will be too long for the frame. Don't panic, just use a washer between the skewer and the chain tensioner, effectively making the dropout/tensioner wider and thus preventing the skewer/axle bottoming out issue.
One more thing: the spacers on the older Zipp discs use the small allen bolt to hold them in place, and it is possible that you may find that the spot where you need to cut the spacer is at, or too close to, the allen bolt. If that's the case, you may find yourself needing to cut from one end or the other so that the spacer is the right length AND you didn't destroy the thing by cutting at or too near the bolt. You will also notice that the spacer rests right next to the bearings, so if you don't cut it squarely, it won't fit flush against the bearing and you'll have a bit of play in the wheel. Don't really have a suggestion here other than to just be really careful and precise when cutting the spacer down to size. If you cut too much off, you can always put a washer between the spacer and the bearing.