O.k. I see now that a 107mm spindle would only give me 2mm. But, wouldn't the RD alone give me 3mm? for a total of 5mm?

It's definitely pretty far off, at least 10mm. It's a regular shimano single-speed freewheel. I've seen that a lot of people here on the forums have used them on their pistas but I don't understand how they could maintain the same chainline since the freewheel is so much wider than the fixed cog. Pretty annoying.