Duster wheel size with mini tub

You can do whatever you want if you jack it up high enough to hang them outside of the wheel wells like those N50's are. No need to move anything except the tail end of the car 3-4" above the top of the tire.

If you actually want the tire to fit inside the wheel well that's a different story. With the springs in the stock locations and not cutting anything, the biggest you can go on a Duster/Demon/Dart Sport is a 275.

If you do a spring offset, either DoctorDiff's 1/2" offset hanger/shackles or the Mopar Performance 3/4" offset (which is really only a 1/2" offset on the hanger with a 1/4" of side loading/cross angling on the spring and 3/4" offset shackles), then you can run 285's comfortably. I run 295/35/18's with a 1/2" offset and another 1/2" cut off the lip on the rear quarters on my Duster, with the car fairly low and the tires inside the wheel tubs. That's the max for the stock wheel tubs, the springs aren't the limiting factor with a 1/2" offset, the tubs are.

If you're talking about a 3" spring relocation, without a mini-tub you're stuck with the same exact 285's or 295's with some trimming as the 1/2" offset. That's just all the stock wheel tub will take, so there's really no point in doing a full 3" spring relocation without doing a mini-tub. Just a lot of extra work for no gain. With a mini-tub, the sky's pretty much the limit. You can fit 335's on a Dart with a 3" relocation and mini-tub, a Duster will have an extra 1" of space compared to that. 355's probably. The quarter to mini-tub inside wheel tub will end up being about 15", so 14" of that can be tire with a 1/2" clearance on each side. That's a 355. You'd just have to work out the backspacing based on your axle width.