I wouldn't advise it cos the underseat tank has a deserved reputation for fragility and cracking. so, too, does the XRL rear frame.
go large at the front - biggest tank you can get! I found the "XRL" likes weight up front, but weaves with weight at the back. if you modify the forks and stiffen the rear spring, you can improve the weave, but sticking a fragile tank back there sounds nasty to me.
the other issue that comes to mind is the seat height and mudguard - the seat is already high and includes a portion of the mudguard. you'd have to consider the impact on dirt going into the airbox, too.