18x9.5 +12
Sorry to drag up an old thread. I'm looking to do a beefy square setup on mine as well. Are you running a stock width rear (do you happen to have flange to flange measurements)? I was wanting to go with 18x10 +25 or so, and if I'm thinking about it right that would put my outside right about the same place as yours. How is your clearance to the inside on the fronts?
So an 18x10 set up, square at +25 is a pretty fair thing to try and do. But it's gonna take significant fender modification, especially depending on the tires you want to run.
My current set up is 18x9's up front with 275/35/18's, with the fender lips rolled and the fenders pushed out with the fender roller, and the lower corner of the fender pushed out almost 2" with an extended fender-bumper brace. That has actually gotten me to the point where I have extra clearance to the fender. So compared to my set up, an 18x10 +25 with the same tire would move the outside another 5mm out by the math. In reality, the difference will likely be larger because the tires are squeezed a bit on my 18x9's. But there should be room for that, if you've done some fender work.
Tomsvaliant was running 18x10's with 285/35/18's on his car, he had a +42 offset but also 3/4" of spacer. So call that +23. His fenders were modified pretty heavily though, they were relief cut, pushed out with a porta-power, and welded back up.
In the back on a Duster an 18x10 is pretty straightforward, again it will depend on your modifications for tire width.
If all you want to do is a 275, then your 18x10 +25 is basically what you need with a 68-70 B body rear end with the stock A-body spring locations. If you want wider than that though you'll need the 1/2" spring offset, which gives you my set up- 18x10's with 295's and a +38 offset.
The 68-70 B rear is 60.125" WMS-WMS, so, if you want to use 18x10's and a +25 with a 1/2" spring offset you're gonna need a WMS-WMS of about 59.125", which is real close to a stock 65-67 B body rear at 59.5". Very doable depending on how much tire and how much quarter work you want to do to make up the difference.
Another option would be to use the 18x10 +25 with an A-body 8 3/4 and some spacers. Just on the math my set up on an A-body 8 3/4 with the perches moved 1/2" would mean an 18x10 +8, with rear disks I have a bit of extra space to the springs. So depending on your tire choice, you could probably run an A-body 8 3/4 with BBP axles, rear disks and a +12 mm spacer which is available off the shelf in a plain spacer (no studs).
So if you wanted to run 275/35/18's square on 18x10 +25's pretty much all the work would be on the front fenders, but it should be doable with just some fender lip rolling and a check of clearance on the rim and suspension components. In the back you could be all stock locations with a 68-70 B rear and probably some quarter lip rolling.
If you wanted 285's all the way around it would be more aggressive fender work in the front, and probably a 1/2" spring offset in the rear. But with that you could run a 65-67 B rear without much else work, or an A-body with spacers.
295's all the way around would be very tough in the front on the fenders, but pretty doable in the rear.