Backspacing on real wheels.

What's the most backspacing u can run on a dually Dana 80 wanting to get my back tires as close to the leaf springs as possible running a 2.6 truck single wheel in the back . Be nice if the Dana 80 wasn't so wide.

I'd measure your flange to the leaf/frame rail and add an inch. That should give you reasonable clearance as long as you're not running a rim that is significantly more narrow than your tire size.

You could use a cab and chassis Dana 80, they are 2” narrower per side IIRC.