The flip/flop wheel is already 120mm wide. The hub would wind up too wide. You could stretch the frame apart in order to accomodate it but then the rim would be off-center.
You can fix the rim but only if you are good with a spoke wrench.
My flip flop wheel does not have any spacers between cone nut and locknut; it's simply a wider wheel. It's not a Velocity but unless someone here has done it, we shouldn't assume you can just convert it into a 5 speed wheel with a few tweaks.
Why not use the flip flop wheel with a single freewheel? Lots of folks like singlespeeds, and your legs will get stronger.
Or, if you haven't bought anything yet, go to niagaracycle or bikepartsusa and buy a 5-6 speed freewheel rear wheel. there's steel (cheaper) and aluminum (higher) but they're both less than a flip flop. The 5/6 is the correct one because there was a 6 speed ultra narrow before they went to 126mm wide regular 6 speed. If you want a higher end wheel, I think Harriscyclery.net has them.