It's not that it's an unusual size; it's just that not many people running less than 20's on modified fullsize trucks these days so the ready-made answer isn't as easy. Better if you compare it to common 20" sizes people are running these days.
Just on a hunch I compared to to a size I used to run, 255/45/20 and they are literally the same dimensions (will vary a bit based on brand and model of tire) but essentially your 255/60/17 is a 29" tall tire with 10" section width. It's recommended for wheels in the 7" to 9" range so an 8" wide wheel is right in the sweet spot.
Now on to the wheel, an 8" wheel with 5" backspace is actually a tiny bit more than I would typically expect from an 8" wheel. It's a good amount of backspacing for an 8" wheel on a dropped GMT400 so that should be fine. 2" drop isn't challenging to most combinations except for really wide wheels or not enough backspacing (like in your pic.)
I'd go out on a limb and say the combination would probably even be fine with a 4" drop; even mostly fine with a 5" drop though you may get some slight fenderwell contact on hard dips just due to tire height.
All of this of course is educated-guesswork because I haven't personally run the exact combo you're asking about. Comparisons to other known combinations and doing a little size conversion using tire size calculator websites, and tire specification sites, goes a long way.
Richard
