I did a 4.5/6 drop on a 79 big 10. 2.5" western chassis spindles and 2" coils, and belltech 6" flip. Truck is sitting perfect level. I also just put 20x9 all the way around. I went very different from the 99% of c10 guys, wider and larger front tire with 255/45/20 and I have no problems. I also studied the backspace for a long time and I think I'm right on.
I was more concerned with the front space as I cared more about getting tire closer to the fender, yet turn and tuck. And most all rims overall width measure 1" wider than the advertised inside dimensions. So my 20x9s are 10" wide, thus...
Front 20x9 bs-5.5" fs-4.5"
Rear 20x9 bs-4.5" fs-5.5"
After comparing my setup, and all the measurements I've seen on others. With a 20x11 you will need to push that wheel to the fender in order the clear the inner tubs. My wheel can, and others have done a 6" fs on the rear. So with a 20X11 at 12" wide and 6" of fs you will need a 6" bs. AKA 0 offset. But I would make sure someone here has done an 11" wheel, it gets tight on the backspace going that wide. As far as the front, I like mine I wish I had a 4.75" fs or even a 275/40/20, get the tire a hair closer to the fender or a little wider tire. I hate the skinny tucked in look, since I have no plan to bag it.
