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

If that's the case, I would avoid the 18x8's with 4.5" of backspacing. You just aren't going to be able to run enough tire. I'm not saying you need to run 275's all the way around to handle well, you don't (although with a Duster you can!). But, if great handling is the end goal I would suggest at least going to 245's. That would still allow you run the same backspace front and rear, as well as being a popular size with a lot of good handling, softer compound tires. Ideally I think you'd still want 5.25" to 5.6" of backspace in the front and 4.75" to 5" in the back, but 5.25" all the way around would work with possibly a small spacer in the back.