The tire rating is stamped right on the side of the tire. The tire manufacturer publishes a load inflation table. It will tell you how many PSI the tire needs to carry a certain weight. Get your axle weights using a truck scale, consult the chart, inflate accordingly.
How will reinforcing the springs add load carrying capacity? How could it not. Whenever a mechanical part is strengthened it gains better ability to do it's job. Swapping heavier duty springs, axles, tires, wheels, etc. onto a truck will in fact increase the ability of the truck to carry weight. It's just a machine and conforms to the laws of physics.
No, it won't change the sticker on the door. Lets say I have a 3500 truck with an 11,000 pound GVWR. My driver's door gets dented and I go to the junk yard and get a door from a 1500 truck to replace it. Now the door sticker's GVWR number is 7,500. I'm I know restricted to a GVWR of 7,500 with my 3500 truck because of what the sticker says?
