big scott the 3 major differences are the motor ss has the k4.6 or the 5.9 and the x4.6 has the f4.6 motor that has a perfect round head. the ss has an aluminum pipe where the x4.6 had plastic. the ss has steel brake rotors and the x4.6 has the red fiberglass. imo the one you hope for is the x4.6 as it has the better motor and brakes.
as for converting it to an xl i would go with an aftermarket chassis plate over the xl chassis the 1/2 inch difference does make a difference on the in flight control. fastlane machine and platinum rc both have nice chassis. and for the dog bones and extra width i would go with http://www.ebay.ca/itm/HPI-100415-S...Accessories&hash=item3f04cfa855#ht_2143wt_952 and offset rims