Don,
I thought I or someone had already replied to this. But it might have been on the Yahoo list. In any case, you can not put a 6" chuck on a 6" swing lathe. If the chuck body didn't hit the ways, it would be very close. The common rule of thumb for chuck size versus rated swing over the ways is that the largest 3-jaw chuck should be 1/2 the rated swing. for a 4-jaw, the figure is 2/3 of the swing.
As far as actually finding a chuck that fits the spindle, your best best is going to be eBay or one of the importers who sell on eBay. Generally, the quality will run from terrible to excellent and pretty much the price will run from what you think is cheap to what you think is expensive. But don't let what you paid for the lathe itself factor into what you pay for a good chuck.. Vintage used lathes by themselves are usually relative cheap. Most of the common accessories are not, at least for decent quality.
