The reason beign that when you convert an old road hub the axle either needs to be respaced or the rim pulled slightly toward the non drive side.
It depends on the way you go about. Easier to just respcace the rear axle.
As kind of mentioned below, you normally have to switch some of the spacers that gave clearance for the freewheel from the drive side to the NDS, giving you a ~42mm chainline, which means you then need to redish.
Cashride: What condition/gearing is the 6sp freewheel in? Are you in Melbourne?
Yeah im in melb, the freewheel feels/looks good, not sure of the gearing off the top of my head, but i think Lats has got his eye on it. If anything changes ill let you know.