I ran that rear size on 20x10 Boss 338's.. 6" backspacing, perfect. I had rolled just a very small portion at the top of the fender lip, which is tough being it's double-wall, and it may not have been needed...just felt better having done it. I drove that truck pretty aggressively and didn't want to risk cutting the tires up.Stock rear end.
No idea on the backspacing.
Told the tire guy what I had, what was done to it & what I wanted.
He handled the rest.
It's a common size on camaro / charger/ challenger etc. Makes them cheap and easy to get used...
245 45 20 in the second pic. Same height as 275 40 20