Weight Distribution Hitch Adjustment Question

If the hitch was set up with the trailer being empty it can make a difference, they really should set it up when the trailer is normally loaded. Setting one link more could make a difference and shouldn't hurt to try it. The perfect setup is to have everything sitting level, the front and rear tow vehicle squat should be almost equal, and the hitch bars parallel with the trailer frame.
Your tire pressure probably should be increased above what the truck's door sticker says due to the tire upgrade. What it should be I can't say.