Origanally springers came at the right length for a stock Harley. When people started chopping and raking the frames the strock length was to short. Depending on how much you chopped and raked you need a 2 over, four over or 12 over. That's where the +2 Or whatever came from.
Our XS's use a shorter fork than Harley does so the -2 or 2 under is about the same length as the stock XS fork. This won't change the ride height or stance much if any.
Most Springers are made to bolt to Harley's or bikes built to use Harley parts. The Harley neck is a bit shorter than our XS's. To mount a springer or most any Harley front end, get a set of the tapered roller bearing kits for the XS650. Install the races in the frame. SEt the bearings on a shelf. Now use the bearings that come with your springer. Put the lower on the stem. put the stem in the frame. Drop the upper bearing in place, Screw the top nut on.
Being the stem is a bit short you may have to shorten the top nut.
Do a bit of research and you will find many threads on how to do this on this site as well as most any of the chopper forums, like the Chopper Underground.
Leo