Hey Matt, its a standard M10 bolt and thread. If you go 1.5" spacer on the bump stop, you will need a m10x75mm bolt. With 3" lifted XK's down here we have been using a 32mm electrical conduit coupler as a spacer (with a 25mm and 20mm inside for strength) which are about 40mm long, and they seem to be a good length.

Although rule of thumb is add length by same amount of lift (inch for inch) but i reckon with the right trimming you can get away with less.