Not to dig in too much on tire pressure, but tires can make a huge difference in ride, and are easier to rough tune than most people think. Tires are designed for a certain contact patch and sidewall give to maximize performance and minimize heat from excessive flex etc. Of course each tire is going to have a different max performance etc, but any given tire and any given load will have an optimal pressure. The max pressure and the max load are the far end of the curve, and the bottom end is pretty close to zero load at zero psi just to have something in mind. If you look at the original tire size, take the front axle weight rating split in two, your front tire pressure will likely be rounded up from the value on the chart (posted earlier, or an example graph below, some of us like pictures more than just numbers). That "Rounded up" is just because tires lose a few pounds much easier than they gain them, which I wish applied to my gut as well. Rear is often the same thing, the max rear axle weight rating split in two and rounded up. Some older 3/4T and 1 ton trucks have an "unloaded" pressure that is an empty bed plus 500lbs of passengers and junk that all of us carry around, then a "loaded" pressure for max capacity. Since new trucks are smart enough to yell at us about tire pressure but not smart enough to know how much crap we are hauling around, they just use the "loaded" pressure. (And these are generally all going to be Cold pressures, they are smart enough to know that tires heat up and gain pressure). Trucks are designed to carry the full load all the time, theoretically, and less people will complain/sue about stiff rides and worn tires than they will blowouts under load that put then in danger.
If you change the tire size on your rig and/or are loading it outside the normal bounds, or even just recognizing that the front end was set up for a plow that you don't actually have, or you always run around with the same load that's somewhere in between, you can weigh your actual truck (4 corners would be great, otherwise weigh each axle), take the actual load as you run it, and probably add a few psi to round up. And of course, everyone will have their own ideal feel, some want more squish and are ok with more wear or sloppier handling, some want more load capacity and cornering sharpness and are ok with jittery rides, and some tires are just going to react different. But overall, using the pressure from the load chart for your tire size and truck weight will give the "optimum" starting point as determined by the tire engineers, for even wear and the best combination of performance all around, and you should tune your tastes to match. Obviously offroad tires aired down for rocks and sand or drag slicks for the track are different animals, this all applies to "street" pressure and typical tires.
Also, as you can see, load E tires basically get their extra capacity from the extra pressure capacity (the load line pretty much continues from the P rated to the higher end of the E rated tire), but when you get lower on HD tires, the stiff sidewalls start to make the curve move. Running 35 psi in the P rated tire below on the front end of a half ton would be massively overkill, since the 4600 lbs of both tires is more than the entire weight of a C10. The chart doesn't go low enough, but 20-something psi is entirely appropriate for that. I had an S10 that was rarely loaded down and so light 2 guys could use it as a wheelbarrow, with wider than stock rims and tires (nothing crazy, but 225/70R14's on 14x7's) that was happiest with something like 16-18 psi front and 20-22 rear (when I did carry stuff) and 12-14 rode great when it was empty. My dad and I both experimented with driving feel, tire wear, and everything to come up with very similar numbers to each other, and when I checked with a load chart (I had to look to find them low enough), they were dead on. But telling anyone I ran pressure in the teens was just asking for an argument, so I just ran it and never discussed it.
Note, some tires have a pretty strict lower pressure limit that has to be adhered to, or you risk pretty bad sidewall damage, but that is mostly in the larger sizes like 19.5's. Big truck tires are pretty well set up for max axle loads, and since not many truckers like to run loads that are lighter than necessary, they can stick with pretty well sidewall max pressure and get along alright. Medium duty trucks can benefit the most from tuning tire pressures, since the loads on drive and steer axles can vary quite a bit on different applications.
Ok, I'll shut up now. Good luck on the replacement springs, I'm interested in how the EZ's and shackle flip work for you...
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