Making the derailleur work with six speeds may require resetting the limit screws. If the hub measures 120/122 over the locknuts, then you probably won't need to do that because the freewheel is extra narrow.
beyond that, if it is 126, you may need to spread the frame apart and then bend the dropouts back with a large wrench so they bear squarely on the hub. Some say you don't need to do that but I experienced slippage when I tried to do without. I would do this even if your rear wheel is bolt-on because the misaligned frame can put a stress on the axle causing it to always run a little bent, and the bearings may wear out prematurely.