How long have you had it?
I did all the snake oil stuff you referred to... Including the kreen. My suggestion is throw in a quart of atf and drive it like you stole it.
Also, I know you say you're certain it's lifters..... But like you've been told before it certainly could be an exhaust manifold leak.
I hope this isn't your problem, but see if my story helps you. I have an '02 with 5.3 that developed a similar problem at about the same mileage or even sooner. The noise got gradually worse. Eventually, I noticed when under load (i.e., towing or fully jammed with college moving, etc) the tap became a knock. Dealer said the oil bypass had failed- closed- and starved the engine for oil, thus damaging a lifter. $3500 later, bypass and engine top rebuilt fixed it. Today, about 50k later all is still well. I shoulda probably sold it and told the new owner not to tow, but I like the truck, so there's that. I've also fixed about everything else on it since then inc fuel lines, brake lines, steering gear,, rust repairs and differentials etc so honestly the engine was just the beginning LOL
