Re: just bought 03 e250 (ford)
I looked into tire fitment extensively with lots of trial fitment and measuring and determined that the biggest I could squeeze in there with 2-3" of lift (and some fender trimming) was a 275/70/17 (32.5" or in your case a 275/75/16 if you can find it) but that is not a common size so 265/75/16 (32") would be your best bet. I bit the bullet and had Camburg install their "Baja Performance" suspension which is advertised at 4.5" lift, I measured mine out to be 5.5" of total lift. With that setup I was able to fit 285/75/17 (34") but I had to trim the front fenders and space the front bumper out 1/2". If you install a coil spacer/ 2" leveling kit IN ADDITION to a body lift, you can probably squeeze in those 305's with some trimming, but they will likely still rub the bumper if you turn and hit a dip at the same time.
I see BFG has a 305/70/16 listed and not the 305/75/16 you stated, that's the same as a 33x12.5" The height will be less of a problem than the width, hence the reason I went with a 285.