Rim backspace help 68

[nacnac]

I have a 68 mustang with a small (2") mini tub and an air ride airbar 4 link. I'm trying to get the most tire I can under my car (goes without saying) on a 20x10 rim. In the pics I have about 11 1/4 threaded bar to represent the overall rim width (11). I went a little bigger to be cautious. The bottom 4 link is the limiting factor way before the inside wheel well so the tubbing didn't do as much as I'd like. Anyway, I'd be happy with a 275 35 but wouldn't mind going wider. My goal is tuck as much tire inside the wheel well as possible and drive it as low as possible . My car is not a race car and will never see a track, so the big tire want is more for looks. I was looking at some inexpensive ridler wheels but they only come in a 5.5" bs so I assume I'll have 4.5" (5") with the lip going to the outside. My big worry is if I get too much bs then the wheels will look too tucked in. I don't want that. I could really use some help.

[nacnac]

And why is a 10" rim with 0 offset not 5" bs but it's 5.5"? Can someone explain that?
and since I'm asking dumb questions, why is a 10" wide wheel actually 11" wide?

[outrgus]

Not sure on the rim questions, but i run a 285/35/18 on my 68 fastback on a 18x9 rim with 6 inch backspace in the factory wheel wheel, lip not rolled.

[PRRC]

Wheel companies measure a advertised wheel width from inside bead to inside bead. The actual rim width from bead to bead on the out side will be about 7/8 wider than the advertised width.