You don't need custom valves, I'm using standard 11/32 "chevy" valves in these heads with + retainers and/or locks and can get 1.75 - 1.8 installed height springs in them. Thus I'd say leave the spring seats alone and get Jim Miller to do a Magnum rocker arm to take care of geometry problems.

The geometry was fairly good on the 520/408 though.