Tire section width is also going to impact backspace/offset selection as well. 5" of rear backspace on the rear rim is possible so long as you don't go too wide on the tire and too narrow on the rim. Excessive sidewall bluge will wipe out your calculations. A 9" rim and 285 tire would be about the max on this and you might still need to juggle some spacers from .062 to .125 to get it just right. I beleive there are a few members here running 5" back space on 17 or 18x10" rims with 275 and 285 tires with no issues at all. Check the fitment database as they are in there.
My avatar has 15x8 with 5" back space and a 295 tire. I have a large amount of sidewall bulge with this combo and it required me to use a .25 spacer to get the tire off the spring pack and it leaves me about a .25 inch to the rear segment of the spring with the spacer, but almost 2" to the wheel lip. Also, don't forget that your leaf springs are wider in the rear than the front, so if your measurements barely fit on the front segment, they won't fit at all on the rear segment.
I also have the same fit up issues with the rim in the front. 5" back space was too much so another .25 spacer for 4.75 backspace is the max I could go before hitting the upper ball joint. However, with a 17" rim you would have an extra 1" of radial clearance so it would not be as critical. I have a 235 tire so there is no problems with rubbing otherwise. you could probably go as big as 255 on the tire up front before it starts rubbing anywhere.