Jack; according to the 68-70 400//350hp., gto shop manual it says to tighten rocker arm ball retaining nut to 20 lb. ft. If this does not solve your problem it sounds like you have Worn OR Dirty lifters & you can remove & soak them in kerosine to remove as much engine oil as possble . Then soak in cleaning solvent for about 1 - 2 hrs., remove & allow to drain & then rinse parts in clean kerosine for a few minutes to cut solvent . Clean each lifter 1 at a time with lint free cloth. check inner & outer lifter surfaces of lifter body individually for scoring,if groved etc.Inspect cam contact surface also.Replace if surface is worn. If you have a SERVICE Manual kit tells you step by step instructions. Try the torque recommmendations & if this fails Buy yourself a lifter replacement Kit. Save you alot of time.