The spring inside a lifter has nothing to do with the lash while running. The oil pressure is far higher than any spring pressure and it is the oil that pumps up the lifter - thus they are called "hydraulic" lifters.
Have u determined whether it is in fact a poly, and not an LA engine?
If there are very many miles on the engine since all gas became unleaded, u very likely could have eroded valve seats. Ive seen new unhardened seats erode to nothing in less than 8000 miles.

Plastic trucks suck...
Iron trucks rule...