I believe that 6 speed freehubs and cassettes did exist but from what I recall, they are in no way compatible with the newer 7 or 8/9/10 speed freehubs and cassettes. The spline pattern was different and the freehub was likely shorter as well.

Mr. Grumpy, are you sure you have a 6 speed cassette and not a 6 speed freewheel? Regardless, if you must have an 8 speed cassette, the easiest fix is to buy a new 130mm spaced 8/9/10 speed freehub wheel (basically any modern 700c wheel) and either flex the stays out by hand or cold set the frame to make the wider hub fit. Assuming the frame is 126mm now, flexing by hand to 130mm is very easy IME. You will need a more narrow, 8 speed chain but your current chainrings will work just fine with it. The width is taken off the outside, not the inside.