I had a problem similar to this with the Cadillac. To clear a 15" wheel, you need around a 11" rotor. Pretty small. Guys were using a Sprinter rotor and still struggling to get it to work and be worth the effort...
Yeah, the problem is, at least what I could find, is that none of the brake companies make an 11" rotor with a 5x5.5" bolt pattern. If you look at how far out that BP is on the hat it is probably for strength that they do not do it. Anyway the brakes I have are a 12.19" rotor and after the caliper is mounted need 14.34" diameter to clear. I spoke with Ricky Bogart and he said the 3 piece (expensive) wheels they make do not have the extra material in the barrel of the wheel like the welded RT's do and will clear 14.34" with 0.200" to spare. So it all goes back to the wheels.