Many other of the same things are posted, Did you use search? But you have to change the interior files out of game in the file itself. Although i have no idea how to acctully do it.
Yes, the cars use the same interior, however, the difference is the roll cage and I'm assuming that is what you want. Unfortunately, you cannot add the roll cage to the XRT or XRG. It's the same reason why we cannot add body kits (who needs them...) splitters, and wings to road cars.
Alright i hope i can find a way to change it but for now i am still searching the web and nothing,and yes i searched the forum first and found nothing that i needed but just a bunch of requests which did not help much.Well thanks for your help,ChooSe[Tech]