This backspacing doesn't sound right!!!

A seven inch wheel is really 8" overall. Look under your car now and see if one more inch of backspacing will still clear everything. If so I think 4.5" BS would be better. Try and measure the outside to outside of the wheels you have now and then see how that size will work or fit with the 8" overall of a 7" wheel. Don't forget that the tires you mount will also bulge past the rim so take that into account also.