On a dart with the offset shackles you can run an 8" rim with 4.75 or 4.5" backspacing.
That is a with a 26x10.50 tire (10.4 section width) If you ran a skinnier tire the offset could be a touch off from that but not much. Say a 1/4" in either direction.
The combo i'm mentioning here is really tight but it works. It is by far the max you'll get under it.
