Don't space out the cog with a washer/spacer. It will cost you thread engagement for the cog and probably the lockring. Stripped threads will cost you the hub and the wheel build and won't get you home. If you have get that cog out, do it by moving a washer in the nut/cone assembly from the right side to the left side, then re-dish the wheel. This will work just fine. Only drawback is that you now have a bastard wheel unlike any other wheel out there.

For my Peter Mooney fix gear project, I built up a wheel around a double sided track hub, spacing it from 120 to 126 with 4mm of dish. Had made for me a double cog "dingle" that I screwed onto the dished in side. Made up a triple crankset. Now I had a wheel with 3 cogs, each one lining up with one specific chainring, giving me three very different gears with the rear wheel moving in the dropout very little. This wheel rides rock solidly in all three gears. Yes, a true bastard, but a very happy one.

Ben