In your case, because the truck is not finished and not at it's final weight, that can be a concern.
If the 4 link suspension is designed to keep the pinion angle the same over the range of suspension travel, then you are fine to go ahead and set it and weld.
If the travel of the suspension has an effect on the pinion angle (like a leaf spring suspension does) then you should have the truck closer to finished weight before welding permanently.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-