Doc, I'm in unfamiliar territory discussing the valve geometrty of the I-6 engines, but when we're approaching the practical limits of lift and rocker ratio on the V-8's, we use a screw-in rocker stud and an Allen screw locking adjusting nut, then we use an adjustable-length "Checking" pushrod to determine the ideal geometry. Once that's established, custom-length pushrods are fabricated. The idea is to reach a compromise between gross valve lift, rocker arm angle, and side loading where the side thrust is mimimized through the majority of the rocker arm travel.
On an engine with a pedestal-mounted rocker shaft like a stovebolt, it would probably be necessary to shim under the rocker stands and fabricate custom-length pushrods to keep the rocker angle and side thrust at the ideal relationship. In extreme cases, it might even be necessary to actually change the distance from the rocker pivot to the center of the valve stems by relocating the pedestal mounting holes in the head. It's much simpler to stick with stock-geometry rockers and do the timing and lift changes with the cam profile. Just about any cam that can benefit from changes in rocker arm geometry is probably too hot for use on the street, anyway. Doing race-engine stuff on street-driven engines is a good way to end up with something that's not much good in either venue.
Jerry
