Barnaby,

Spacing the 120 to 126 will be fine. Because I was using a 110 on a MTB, using a 107 BB wasn't enough and I had to use spacers between the chainring and crank, which bothers me. I do like the small 'Q' factor, however.

The Phil Woods come pre-spaced but I don't think that they flange, cog or bearing spacing is different for the 120/126/130. This would add a lot of cost and it really isn't necessary. Just the fact that the wheel is symmetrical gives you plenty of strength.

Your original plan sounds the best and you seem to know what you are doing. Now that I have decided that I really like the fixed gear, I also will get a double fixed 120 and space that.

Have fun!