You will NOT need to "back off on the WDH."
Remember, the WDH has NOTHING to do with rear sag on the truck. The entire purpose of the WDH is to push the front end back down so you can steer and stop. Raising the rear end is just a side effect.
Even if the truck doesn't squat as much because of the stiffer springs, the front end still needs to be pushed back down so you can stop and steer.
Whether you will need to adjust the WDH depends on how much the new springs raise the rear end. If it raises it enough to lower the head on the shank by an entire hole, no adjustment needed. If it only raises it part way you may need to tilt the head back a little more to regain the proper front ride height.
However, rather than adding helper springs, consider taking the truck to a spring shop and having new springs put on, or getting a new set and putting them on yourself.
My issue with helper springs is that they put additional stress on the spring eye, and on a truck as old as yours, that area's already stressed.