Opinions wanted: '75 Duster with 18" Boss 338 wheels 4.5"backspace

What's the planned use for the car?

18x8 with only a 4.5" backspace is a terrible waste, that's my opinion anyway. With that backspace the widest you can go up front is a 225. It will fit fine out back, unless you figure that you'll have a giant cavern of a wheelhouse with a skinny little tire in it.

A duster with a rolled quarter lip will fit an 18x9 with a 275 out back with the proper backspacing, no problem. Same for the front. You don't even need rolled quarters or fenders for that. The problem is the backspacing needed isn't the same front and rear if you have an a-body 8 3/4 rear axle.

If you want to keep the backspacing the same, you could run an 18x8 with 5" to 5.25" of backspace. You might need a 5mm spacer in the back if you get to 5.25". That would let you run 245/40/18's all the way around. Wider than that and you need to stagger the backspacing front and rear. 255/40/18's are 26" tall and have a lot of great tire options, but you'd need an 18x9" and at least 5.75" of backspacing in the front, but still only 5" to 5.25" in the back to stay off the springs. Plenty of room to the outside in the back though.

But that's only if you want to run wider tires all the way around. I run 275's on the front of my Duster and 295's in the back, but I have a 1/2" spring offset in the back (and a b-body 8 3/4).