Question about replacing 15x7 wheels with 15x8

Your thinking is correct. Increasing the backspacing by one half of the increase in wheel width should keep the tire in exactly the same place in the wheel tub. The increased wheel width can make the tire section width a bit wider, making it come a little closer to the outer portion of the wheel tubs & the leaf spring. If you have a lot of clearance on each side you snould be fine. I have run tires as close as 1/4" to 3/8" from the spring & only ended up with a shiny spot on the spring. I would try to stay at least 3/8" from everything, & more is always better. I read somewhere that you should have half an inch.