I agree, the head casting will always be a restriction. In quite modern fashion, the intake valve is very large (area per inch of displacement) and well placed (not near the cylinder wall) exactly as John Kaase suggested. The valve also lifts into open chamber volume with no masking, so more lift is not as useful as in a shrouded chamber (such as Gen-3 250).
However: Spintron has made a liar out of much trust in pushrods, especially big block (BBC, BBM, BBF) with tall decks and 10-11" pushrods. Even 7/16" are willow wands with big valve springs. IMHO much of the success of the LSX is due to the raised cam CL making the 7.4" pushrods nearly immune to bending. Crude math: a 7.4" pushrod (all other variables removed) is about 280% as stiff a 10" pushrod.
My intention was to allow much higher rate of lobe lift (inches per degree of rotation) to create a larger "window" without late IVC closing and big OL. The .990" tappet face makes this possible (as opposed to the .842" Gen-3 tappet).
Making an intermediate pushrod resting inside the head's relief hole allows the lower pushrod (which only reaches from the tappet cup to the block deck) to be less than 1/2 the original length, which increases stiffness 10 times vs. a 5/16" replacement - bending is no longer present.
Another factor is use of (aftermarket, obsolete) high ratio rocker arms such as B&B, which locate the rocker pushrod adjuster closer to the shaft, which reduces maximum pushrod diameter. Using an intermediate allows an upper pushrod to be much smaller for clearance with bending risk.
