Put the back tires of the truck on the ground and jack stands under the front axle so that the load is only on the axles just like it would be at static ride. Then take your angle finder and measure what angle the frame rail is to the ground say its tilted to the rear 3*. Then measure with the angle finder again across the top of an inner C front to rear to see where your caster angle is. Say you set it at 5* then you level the truck out from 3* to 0* now your caster will be 2* if that makes sence.
In your pictures it looks like with the pinion that high there's no way you could have a good caster angle. Remember positive caster (Good) is when the upper bj is behind the lower bj. I'm gonna guess and say that with 4" waggy's your not gonna be able to get more than around 4* of caster if you want to keep front driveshafts in the truck. The only other option would be cutting and turning the inner C's
I ran into some pretty big issues doing this same swap on an isuzu trooper a while back. In my case the passenger spring perch was cast into the diff housing so I was pretty much stuck with my caster/pinion where ever the spring angle came out. I ended up with a 23" long front driveshaft that at ride height was a 42* angle. It was a mother and unfortunately I think you may have similar problems due to the tall springs.
Basically you need to forget about pinion angle and worry more about getting the caster right.
