The correct amount of caster depends on the vehicle, 4 degrees is just a good starting point. If the rear of the truck is a lot higher than the front, caster changes, same for different tire hight front and back. Add a load of sand in the back, caster changes, what you want is a decent driving truck with and with out a load so there is some compromise for each case.

Setting on level ground, smooth concrete like a garage floor works best, turn the steering wheel full left and full right, the front of the truck should lift up at each direction and fall back down when centered. The amount of lift is due to the caster shims. Too much caster and you are lifting the front each time you turn, too little and the truck won't return to straight ahead after making a turn and will feel very loose going down the road. You will be very tired of always making corrections to keep it straight. With too much caster, the steering will feel heavy going around corners and it will want to return to center as soon as you let it, but they will go straight.

Be sure to check the spring bushings, if they are worn, the whole axle assembly will always be shifting around. It won't hurt it to be a little loose, but with a good tight setup, your truck can drive just as good most modern trucks. Having the steering box adjusted correctly is your next step, start another thread or search for adjusting procedures on how it's done. Adjusting one is a DIY job if you follow the instructions, its well worth the time and effort, and in my opinion, one of the best things you can do for a better driving truck.

1937 Chevrolet Inline-powered Streetrod