I really think that in order to clear a 275 up front you need a little more than 6" to keep it out of the fenders. Especially with a 275/40/18, as that's a pretty tall tire for the front.
With a B body rear and a 1/2" spring relocation I think you can pull off the same backspace front and rear with a 275 at around 6.3-6.4, which is a pretty common offset. It'd be close enough that if you had issues you should be able to use a hubcentric spacer for adjustments, nothing big.
You won't fit a 305 into the stock wheel tubs. I'm pretty sure that even a 295 would be too much if you're lowered, the wheel tubs are too tapered. When I had the 275/40/17's on my Duster in the back they just barely fit, it would probably have needed a quarter lip trim for the street. A 1/2" offset buys you the rest of the wheel tub, but that doesn't get you a 295, the section with is over 3/4" wider.
Without a mini-tub, I think 285 is the best you can do unless the rear end is jacked up in the air. The spring offset or 4 link will only get you to the wheel tub unless you mini-tub it, and that means no 295's or 305's.
