biggest tire size

'64 Cuda said
An 8" wheel with 4.5" backspacing would probably allow for a wider tire than a 7" wheel with 4.25" backspacing. The same size tire on the 8" wheel would actually move away from the spring by a bit more than .5", depending on variations in the cross section of the tire due to the different wheel widths. You're moving the inside edge of the wheel .25" closer to the spring but the center of the wheel, as well as the center of the tire, is moving away from the spring by .75". I think too many people get hung on backspacing measurements. It's the offset that determines where the tire sits in the wheelwell.

Whether you use offset or back spacing you get to the same answer. It just more common to use back spacing when you are talking about muscle cars. The term offset became common with FWD cars that have very large back spacing for the rim width. Zero off set or back spacing that is 1/2 the rim width centers the rim/tire on the plain of the wheel mounting surface on the axle. In your example the 8" rim with 4.5" of back space is .25" closer to the spring than the 7" rim with 4.25" of back space.

Second generation Barracudas, Dusters and Demon/Dart Sports have relatively large wheel wells and its the location of the leaf springs that limits the tire size before the actual wheel well.

In my case above I have a 8" wide rim with 4.5" of back spacing, that is .5" of positive offset. With a 255/60 tire mounted the tire comes within 1/2" of touching the leaf springs. By going to a wheel with 4" of back space, zero offset, I move the entire rim/tire outward 1/2" away from the spring which would give me 1" of clearance to the rim and about 1" clearance to the wheel well lip. A 275/60 tire generically has a cross section that is .8" wider than the 255/60 or .4" per side or .6" of clearance on each side.