welcome to the forum. i have heard of people using the xj springs. you are limited on the rear though due to the gas tank. if you wanna lengthen the rear that much you'd have to relocate the gas tank, or i think genrite makes a gas tank for a ton of money that gives you more room. for an inch or two they make offset spring plates. you can also try to get a little out of the front if you are just going for a little more wheel base.