71 dart 17 inch wheels 5x4 bolt pattern

I'm doing the same on my Barracuda and have measured, measured and measured again. The stock 8.75 allows for 5.5 inches from the face of the drum to the edge of the leaf spring, then 5.5 inches from the face of the drum to the fender lip. There is a little more space than what I have said (up to 1/4 inch each way)but I'm safe using these measurements. Now you have to consider your backspace on the wheel and tire size. Tire size specifically looking at how much it will bulge beyond the wheel rim. For instance a 255/45/R17 is 10" wide. It has to mounted on a minimal 8.5" wheel but to make the math easy lets mount it on a 9" wheel with a 4.5" backspace and 0 offset, thats a dead centre wheel on your drum. Your 10" tire would extend to 5" from centre of the wheel, evenly inside and outside. With a 5.5" space both inside and outside you'll have .5" to spare on each side with a stock 8.75 rear. Now the tricky part, you could go with an 8" wheel and split the wheel dead center making it a 4" backspace. However that will actually push the tire/wheel more outside, when you really want to keep it tucked in the wheel well. So you still use a 4.5" backspace and 0 offset. The max tire for an 8" wheel is a 245/45/R17 which is 9.6" wide. Here's the math again, take the total tire width that extends beyond the wheel width and divide it by two (9.6" - 8") /2 = .8" or 13/16". Add that 13/16 to 4.5" backspace and you get 5 5/16" which is just under your 5.5" allowance.
As for the front; Its similar for backspace of 4.5" centred in the wheel well but not as forgiving on turn radius for a wide tire. So an 8" wheel likely will only handle a 225/50/R17 tire. A 7" wheel can handle the same tire 225/50/R17 but the backspace should be no more than 4.25". In all cases the backspace is based on a 0 offset.
These are my calculations thus far. You can query Tiresize.com to get the measurements of tires I've given you. Btw I'm looking at a 25.7" to 25.9" wheel diameter, which also affects the turning radius on the front.
Hope this helps