18x8.5" is doable all the way around, even 18x9" is doable all the way around. The problem is going to be using the same offset.
The B-body rear helps, but to maximize tire size up front you'll want around 6.3-6.4" of backspace. That should let you run a 275 up front. But to get the 275 out back you're only going to want about 6" of backspace, even with the B rear. So, to run the same offset you'd have to think about spacers out back. 1/4" is about as big of a spacer as you want to run, unless you're talking about the spacers that include a new set of studs. But those only come in 1" or bigger. So...
6.25" backspace is probably the answer. Should be enough to get 275's in the front, and you can run a 1/4" spacer out back if you need to.
Do you have the 1/2" hanger/shackle spring offset on that B rear? That could be the other answer. I know of a Demon with a B rear running 18x9's, 6" backspace, and 275/40/18's with the springs at 43". Move them to 42" and you can make that 6.3-6.4" offset work all the way around...
