I would also put a high volume pump in it...just save the new one for the other engine. If you can get the oil pressure up to 40 lbs idling at running temp that should keep even the weakest lifter from tapping. I don't know what else to tell you. If you do swap the pump to a hvp, you can check for shavings or metalic powder...hopefuly there is none. You can try to change the releif valve spring to a high pressure one if yoy don't want to spend the cash on another pump. All this only if ur sure it's the lifters loosing pressure and not piston slap or something.