How to recognise a 6-speed freewheel compatible hub?
All freewheel hubs have the same thread and are compatible with single, 5, 6, 7 and maybe some 8 speed freewheels. Position of the hub is set by dishing of the wheel and spacers on the axle. It's an interesting question. But I think that you should worry more about the rear spacing. 6 speed modern freewheel is not considered to be wide comparing to ones with more cogs. I wouldn't worry about this, just in case stay away from track hubs which are designed for single speed freewheels. I'm upgrading a 1980's ten speed bike now by replacing old 5 speed freewheel with 7 speeds to match new indexed 7 speed shifter. Previously I put 8 speed freewheel on that wheel but I had to add spacers to spread the seat stays a bit. It worked with original friction shifter but I don't like it.