There are shims for the valve springs to ensure that the valves, valve keepers and retainer gives the correct valve spring installed height.
With the factory stamped rocker arms, shimming between the posts and rocker shafts is needed sometimes for high lift camshafts. Stamped factory rocker arms are fine for ~.500" lift. Bigger camshafts (above .500" lift) really need adjustable rockers to maintain proper rocker geometry. From the one picture with a few of the liters showing, I'm not seeing a crazy highlight camshaft.
By the way, all Chrysler V8s had solid pushrods until the 5.2/5.9 Magnum engines. Even the mighty slant 6 had solid pushrods (and solid lifters) for the longest time until the factory used a hollow pushrod to convert the slant 6 to hydraulic lifters!!
I think your new rocker arms will solve a lot of your issues.