I have two 600 rears, one is UG and one is HG, both 7-spd and at 126mm. You'll need to swap the freehub body and ideally, the axle to go 130mm. But Sheldon Brown's site stated it's doable with the original axle. Cost depends on where you source the 8-10 spd body. One kit just sold on ebay yesterday for $7 complete with road and mtb spacers and dust caps. High side about $35. If you have a co-op nearby, check the bins for a donor freehub body.
And yes, the 600 hubs are very nice quality. But in the end it may be wiser to buy a new set of hubs at $85.

Oh, and HG is better than UG. But does it matter? Probably not enough. UG should have came in 30 and 32t....I have a UG freewheel (not cassette) with a 32t low.