Wow. Holy thread resurrection batman.
And, BTW, your numbers aren't right. First off, stock suspension for a '72 Duster/Demon would include SBP brakes, and you won't find any of the rim dimensions you listed for SBP. If you did, your offset numbers don't match the narrower track width. As for the wilwood brakes, they have several options and not all of them result in the same offset.
You will not fit a 285 on a 10" wide rim up front without significant fender modifications. Not happening. I run 275/35/18's on 18x9" rims with a 27mm offset, and I am at the limit at both the fender and the frame. I've already re-worked the lower front fender corners and braces, and used all of the adjustment in the fender mounting to get enough clearance. Tomswheels ran a 285 on an 18x10 on the front of his Valiant, but he also pushed the front fender out with a hydraulic jack. 25mm to 35mm is the offset needed for 9" to 10" wide rims, depending on your brake package and car. 35mm will usually require some kind of increased offset with brakes and possibly a spacer. That's using 73+ disk brakes as the reference point for the offset.
In the back you won't get away with an 11" wide rim without cutting stuff unless your car rides as high as a 4x4. Remember, rims are measured on the inside lip where the tire bead mounts. An advertised 11" wide rim actually measures 12" on the outside edges. The rear quarter lip can only be trimmed back about a 1/2" before you get into the spot welds holding the quarter to the wheel tub. Cut any more than that on most cars and you'll have to reweld the tubs to the quarters. I cut mine back a 1/2" and still lost a few spot welds in the process.
+15 is about where you need to be in the back with an A-body 8 3/4 with BBP axles. Since you're talking about 0 offset, I'm going to assume you don't have an 8 3/4 (7.25" and 8.25" rears are about 1" narrower than the 8 3/4).
A 285 might be possible on a stock car, but only one that ended up with generous clearances or that doesn't sit at a lower ride height. On my Duster I was maxed out with a 275 when I had the stock spring locations and uncut quarter lips.
Currently I have 295/35/18's on 18x10's on my Duster. That required a 1/2" spring hanger/shackle offset (42" c-c on the spring perches) as well as a 1/2" trim on the quarter lips, and I'm still maxed out for clearance at my ride height.
