pspence, If both rear wheels fit the frame alike, then there is no problem with spacing. If you are going to use a 7S cassette, you will have to install a 4.5 mm spacer before installing the cassette. No spacers are requitred for an 8S or 9S cassette, but you may need to install a 1 mm spacer if going with a 10S cassette.

I maybe incorrect, but I think the old free hub is an UG (Ultra Glide) and the cassette for it will not work with the new free hub as it is a HG (Hyper Glide).

Brad