Lifter problems????

40 psi seems a little low. Seeing as it's losing prime, I'd suggest a couple things. First, pull the oil pum and chekce the pressure releif/by-pass. It may have soemthign in it sticking it open. Even stock engines had pressures in the 50psi range over 2K rpm and higher if the bearings were in good shape. Second, make sure the oil filter you are using is a high quality one. The $1.99 filters at Wally World are worth the price. A good filter will cost twice that at least. I use Wix, or Napa Gold, or Car Quest, all of which are made by Wix. Your pressure will drop with most synthetics, because they run thinner than conventional oils of similar wt. Some have higher pressure, I find Syntec does anyway than a similar Dino oil. The protection should be more than enough with the 10-40 Amsoil, but you pressure IMO is too low. If those other two ideas dont fix it, stick an HV pump in it. Dont worry about draining the pan, just keep the level at "full". The noise is an indicator of a problem, but the lifters need a bit of oil pressure now and again to stay pumped up. I'd look into this stuff first.