I'am running a 225/40 18 on my front with a 4" backspacing, on my 68. This is the best backspacing for this size tire wich measures 9.1" wide section width. When turned all the way the tire will rub the frame slightly but it's nothing that will do any harm. The car sits as low as I can get it without rubbing to much. But it will rub the inner fender when going down the highway and hitting a big bump. Right know my front springs are original 6 cylinder springs, but the car has a iron-head big block, the rubber stoppers are about 3/4 inches from hitting the control arm. Soon I will be replacing the springs with stiffer one's and I will also replace the factory rubber stopers with shorter ones. As for fitting a 255/45 17, which have about the same diameter but are about an 1"wider section width. I would say using a 4.25 backspacing and you may not be able to turn as sharply, or have the car as low as you would like, your call.
As for the rear mounting a 275/40 tire which measures at 11" section maybe a challenge. You might want to go with a 5" backspacing, or a little less. Your limiting factor will be the frame. I would suggest that if you know of someone with these size tires, to put them on your car and go from there, by measuring the clearence. I have a 275/35 20 on my rear with a 4" backspacing, and from the looks or things I think I could of went with a 5" or 4.75" backspacing. When I look at the rear tire the first thing that will hit on the inside will be the frame then the shock and then the spring. The tire/wheel combo is over kill, and to wide and I soon will be selling the combo(at a great loss, origanally paid about $4000 for the tires and wheels) in favor of some 17" myself. Hope this is'nt to long and gave you some help, let me know what you think or have any questions. Good luck.
68 468 700R4