If it has a 7.25 and a 340 and you put a set of 275's on there you can probably measure the life of that rear axle in hours. One good launch and that extra tire will probably dump the rear axle guts right in the street.
With a 15x8 and a 4.25" backspace you should be pretty close to the right spot with a 7.25". If you need a wheel spacer it will be a small one, 1/8" at most. I would mount up the wheels first, then see if you need anything at all. As for the distance from the top of the tire to the wheel well, that will depend on what springs you have and how worn out they are.
