If they've been there that long I bet somebody there will know exactly what to do, but you may not even need their help. Caster angle is simple to measure...adjustment on these is done only by using shims and the spring width on the AD truck was 1.75". That width isn't too common today but on Amazon you can get a pair of 2 degree made in the USA shims for $32:
https://www.amazon.com/WFO-Concepts-Steel-Degree-Shims/dp/B09B2P864D?th=1Those are what you'll need. If you have new springs and a good (not damaged nor bent) axle, those springs and those shims will get you really close to an ideal measurement.
