OK on the pushrods. Yes, that is my favorite theory right now.... running out of everything else! How things can change on the pushrod lifter side:
Stock shim head gasket to Felpro kit head gasket: .025" on pushrod
.005" wear in the rocker seat and shaft: .007" on pushrod
Valves tipped .010" or new seat installed and not set at the same height (.010" is not too hard to be off) : another .007" on pushrod
So that is .039-040" change on the lifter/pushrod area without any manufacturing tolerances on the pushrods, rockers, lifters, and head castings, or wear on the pushrod tips, rocker tips or cam. And the cam base circle can be a bit small too.
So possible. Is it this? IDK.... Put that lifter on the base circle and see if the pushrod spins free. That might be a clue.
Or swap around a pair or rockers to this position temporarily and see if it gets better or worse.