Technical king pin angle

Great question. My experience has shown that the castor changes very little if you wait to set it after you have all the weight on prior to setting it. This can be a bit tricky on axles that have perches cast into them like most I beam axles, vs. tube axles you weld the perches on. But if you go a little more than the typical 6-7 degrees it will fall within range once weighted. That is if shackles are at the rear. Shackles at the front will gain angle or castor when loaded, so setting them one degree under usually works.
I simulate weight by stacking bags of sand or cement on the front if I don't have the engine/trans in. But whether it's a tube axle or a I beam, I still don't weld anything up solid until I'm as close to full weight on the build. Plenty of small tacks are all I do, and once shackle angle, castor, and everything looks fine, then I go back and burn in my welds. Things like the front clip, or other lighter items wont add enough weight to worry about, and I've never seen a marked change in castor after bolting on the front fenders, hood, etc.