One of the members here is running 6.5" backspacing on his 15x10 rears with 295/50R15 tires. I have 5.25" backspacing on mine, and that's leaving almost 2" to the springs, which tells me another 1.25" of backspacing would have the inner edges of the tire/rim pretty much right there on the springs.
You might want to check with the seller - the 9.5" rim with 6.25" backspacing could be a typo, considering the 9" rim only has 4" of backspacing. A full 2.25" more backspacing might look a little goofy with the fronts being right about where they should be and the rears tucked in really deep.
Hope this helps.