From my experimenting, it's not the disc that fouls, it's the caliper. I found that, anyway. If you have 13's that clear the caliper, they may have some funny backset (like off a front-wheel drive car)? If they fit, get them balanced and see how it goes. Sometimes odd-seeming combinations will work fine.
I saw one only XP Futura with 13s and 4 stud discs (almost ten years ago in Surry Hills), and even today am not sure what they were off - certainly not 1965 Ford Falcon issue.
BTW, I am mentally chewing through splash shield issues. I've an XT spindle that has a factory cut-down splash shield (GT?), and am toying with either patterning off this for the XP shields, or beating in some kind of louvre/scoop to aid cooling. They're not a huge brake rotor and extra cooling appeals for that reason.
Regards, Adam.