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

You should be able to run a 275/60/15's.

That's basically the same size as you can run with a 1/2" spring hanger/shackle offset, which lines the springs up with the stock wheel tubs. Without a mini-tub, the 3" offset doesn't gain you all that much clearance because the stock wheel tubs are still in the way.