[Wbulk]

You are right. Once you tell them what back spacing you want you own them if they are custom made, at least that's the way mine were from Team III. I really don't think you can go with 8" in the front without some rubbing problems. You didn't say what size tires you were going to use and do you have the B and B narrowed 9"?

I can tell you what I went through as I spent a long, long time, on this issue. I am still building my car but have mounted the tires and wheels on the car when the body was on to see how they came out. I read all the threads on the subject to, but I felt it was to risky doing what so and so said as taste vary, and I have seen some bad setups. First I had to decide where I wanted the tire to set in relation to the fender lip. I wanted the bulge or section width of the tire, (not the tread), even with the outside edge of the fender lip and I wanted this to be close to the same front and rear.
FRONT
For the front I felt there really wasn't much of an option. This being that 4.5 back spacing was the greatest backspacing Team III could make them for the fronts. If you go with 8" wheels on the front this will push the tires outside the fender lips and you may have some rubbing. I also went with 235 tires on the front. For the fronts I believe B and B recommends 7" wheels with 4.5 back spacing, and 225 tires. I layed it all out on paper including the rim mounting surface, fender lip, wheel mounting point with the camber correct, and got the specs on the tire midsection. I ended up with 7" wheels with 4.5 back spacing. This lay out on paper really requires attention to detail and no errors. Keep in mind that a 7" wheel is the inside so it is really close to 8" outside to outside. Back spacing is wheel mounting surface to backside outside of the rim. As it turned out my fronts turned out pretty close to what I wanted.
REAR
For the rears you are right back spacing for 8" and 10" wheels does not work out the same. In an ideal world the body is centered in the frame and the axle is also centered in the frame. So you need to check both on each side to see where you set. Again, I got the measurement from the fender lip to the wheel mounting surface on the axle, and layed those two points out on paper first. Once you have that mounting surface you can lay out various wheel back spacings and then enter in the section width of the tire. Then you can see where the tire will come in relation to your fender lip. I used 10" wheels with 295 tires. The section width of the tires was 12.5". Advertised section width and actual may be a little different, and the same tire mounted on different width wheels will effect section width. For my taste 5.25 back spacing is just about right. Keep in mind many wheel companies only go to 5.5 back spacing for a 10" wheel. Also, if you have the bump stops on the rear the tire will hit the bump stop before the tire could hit the body.

These are just some ideas you might consider.

Wayne