Another wheel question - leaf springs moved inwards. How big can i go?

275/60/15's are usually only recommended up to a 15x9.5" rim. Typical is 15x8 or 15x9. It should work, but the tires will be a little stretched. I'd much rather run 9" wide rims with that tire.

With a 15x10" and 5.5" backspace your clearance on the quarters is going to be TIGHT. You're going to have to be spot on to run 275's anyway. Ideally, you'd probably want about 5.75" backspace on a 10" rim. Better to have a little more backspace than a little less, if you have too much you can adjust with a small spacer. Too little and you're stuck.

If it were me, I'd be looking for a 15x9" with 5" to 5.25" backspace or a 15x8 with about 4.5" to 4.75" backspace.