Take this with a grain of salt because the only experience I've had with hydraulic lifters was in my old HD shovelhead. I did find out first hand there that a tiny piece of dirt or crud will keep a lifter from pumping up, resulting in constant lifter clatter. Since the truck sat for so long some crud may have come loose and contaminated your lifters. Just a thought.
Another thought (I do that once in a while
) the hydraulic lifters on my old HD were directional, meaning that the oil hole in the lifter had to be on the same side as the oil passageway in the engine. Many times I helped "fix" noisy lifters by turning them around after an unknowing person put his lifter(s) in backward.
