No, you don't add lifter oil additives through the valve cover caps on a 1972 big-block Chevy 396 (or any engine for that matter). The valve covers provide access to the valve train, not the lifter galleries. Oil additives are added to the engine's oil *itself*, through the oil filler neck, usually located on top of the engine near the valve covers but separate from them.
The oil then circulates throughout the entire engine, including the lifters. There's no need to add anything specifically "to the lifters" or to different "sides" of the engine.
