Mike I debate this too but my gut is to go 3/8, then again there are lots of SBF that go 8500 lus with 5/16's with a lot more spring and lift than this one. 220 seat/500 open. A shorter pushrod for a given cross section is stronger too (in compression). The higher the lift the longer the pushrod length, the more extreme the arc. The Achilles heel with the SBM is the 59 degree lifter valley angle which imposes more lateral forces at the lifter side tip and (applying trig) must also slightly reduce the net lift at the valve.

One thing I do pay very close attention to is the the rocker spacing on the shafts, I use shims to keep the lateral motion between the rockers on the shafts to a minimum which in turn helps minimize a lot of pushrod oscillation. On small blocks I also like to run grooved #2 and #4 cam bearings to keep constant 360 degree oil flow to the shafts