valve adj. on engine while on engine stand before. start up

Since '66 still used solid lifters from the factory, I'll assume you've converted to hydraulics; and since you're asking about setting lash (actually, preload with hydraulics) I'll assume you retained the adjustable solid lifter rocker arms. I'll also assume you got custom pushrods of the correct length (solid and hydraulic lifters use different length).
Prime the engine, and then adjust the rockers so there is just zero clearance. Then give the adjusters another 1/4 turn.