Since your running new aftermarket spring's there is not much you can do to lengthen the wheelbase...You can redrill the spring perches and gain an inch at each axle,or cut off the factory mount's,move them to desired wheelbase and weld back on,then remount the shackle hanger's.
If your worried about height would going back to SPUA be an option??? With your new 1.5 inch spring's,add longer shackle's and some fender mod's or a small body lift should have you running up to a 32" tire.