Thanks for the replies so far. I messed around with the bike/wheels and found it fairly easy to pry the frame onto the wheel--I definately didn't flex it 10cm, but it wasn't a "natural" feel either, so I'm guessing the spacing is 126mm. It's not too much to do every time I pull the wheel off.
So, now I have to figure out the cassett and gearing. Is it possible to put the old 6 speed cassett on the new wheel and use the same chain? Would some type of spacer be necessary if I did this? I don't feel like I need more than 12 gears unless it makes the switch easier for some reason.
And as for the BB and cranks, would a modern Ultegra crank work? (I found one in good shape on CL for $30).