It's a whole changeover mate - you need to source some EL GT Calipers, stub axles and 329 mm rotors (well pretty sure they're 329 mm according to my sources...but I'm tipping 330 would be ok ??). To answer your question - an aftermarket twin piston setup (ie CEBCO) would fit your XR 16" rim AFAIK
If you're looking for good BFYB, I'd lookup cebco brakes - they boast they're the only place around with twin piston kits that are ADR approved. The kit includes twin piston calipers, 330 mm rotors, steel braided brake lines and stub axles. This is not fitted though, and I think it's a changeover price on your stub axles too form memory. The kit is just short of $2K too.
They're located in Melbourne (I can give you details if you want), but yeah if you do look them up - the guy there John is a top bloke !