18" wheels

Correct, you have to change the width of the rear axle. And it depends on your spring location too, that changes what you need for rear end width.

If you have a 1/2 spring offset and stock tubs The closest fit to match the offsets front and rear with wide rims is probably the 65-67 B body rear axle. I have a 68-70 B body rear in my duster and my front offset is 27 and the rear is 38. The 65-67 would be pretty close to even with the max width rims, a small spacer in the rear would make it perfect.

The other option is to just not max out the front tire width. If you stick to an 8" wide rim and 245's you can run a 5.25" backspace all around. With a 1/2" offset you wouldn't need a spacer in the back, with the stock spring locations you'd need a spacer.