Did I get some bad advice?
Yes. You would really need a 6" rim for that tire, 5.5" at the minimum.
You mention about the 345s on 6" rims. That might be alright at low speds, but once you get into highway speeds, well, that's a different story. I speak from experience on that. Had to diagnose a wander condition on a Jeep with 31x10,50s on it, mounted on the stock 6" rims. Suggested 8" rims to correct it. Once we mounted them up on the wider rims, the handling problem was corrected.
