Lots of false information regarding tire pressures when going to an LT tire from a P metric. Definitely NOT the pressure on the placard, you'll kill both your rolling resistance (MPG) and the outside edge of your tires. Toyo made a chart specifically for this, and is very helpful.
In short, look at your OEM tire size and pressure on the door. For me it was
285/45/22 @ 36 PSI = 2416lbs. I made the switch to a
35x12.50x20, so I take that 2416 to that tire size which takes me to 43ish PSI (right between 40-45). I run the front at 48 being a little heavier, rear at 43. When i am towing, i bump the rear up to 50PSI.
If your stock tire size was 275/55/20 @ 35psi (just guessing) that would equal 2403lbs on the chart. Take that 2403lbs to your new LT275/65/20 which gives you 2475 @ 45PSI. So you'd want to give your tires 45PSI for the right load. Now you can adjust a little bit since these are trucks and not a 50/50 weight distribution, you should run a little higher in the front than the rear. Maybe 50F/45R. All depends on your gross axle weight. Every other method is really guessing, the chalk test when brand new can be helpful however can be tough to get truly accurate.
Hopes this helps.