Cam and lifter problem

Ok got a problem with clattering lifters in a 454 Chevy I rebuilt for a buddy. I know it's not a Mopar but I also know alot of you engine guru's know more than just Mopars. Here's what we did.

I totally rebuilt his 73 454. Hot tanked, checked all clearances, turned crank, new pistons, bearings, new oil pump, rebuilt the heads and set the valve spring height, the whole 9 yards. I double checked everything I could think of. The lifters fit good and tight in the bores also. I installed a new Comp Cams XE268 cam and used Sealed power lifters (these were the lifters Northern sent, I was going to send them back for Comp's brand but they swore they were the same product so I used them). I used plenty of cam lube. We got the engine in and fired it right up and took it right up to 2200-2300 rpm for about 1/2 hr. BTW: We used the recommended Rotella oil. Shut it off and changed the oil. 80 very easy miles later a lifter started hammering and we checked it and a cam lobe was going flat so we yanked it back apart and installed another new XE268 cam and lifters this time using Comps brand lifters. Changed the oil before ever starting it and installed fresh Rotella oil and a new Wix filter. Fired it up, ran it for 30 minutes at 2200-2300 rpm. Shut it off and drained and changed the oil. Now with the Comp brand lifters the lifters clatter at idle after the engine is warm. My buddy ran them again and they were set right (we have the preload set at approx. .030-.035 per Comp Cams recommendation). The oil pressure is 35 at idle and goes right up to 48-50 as soon as you give it gas. We put another Autometer gauge on it to make sure the first was reading correctly and got same reading withing 2 lbs. Were about to pull our hair out with this thing. I've done everything by the book and it still don't work. At least with the Sealed Power lifters they didn't all clatter when it idled.

Tried calling Comp Cams and their tech line is a joke. I was prepared for them having all the numbers written down. After we told them everything we had done and it was as they recommended all they could say was we had to be doing something wrong. They don't make bad lifters. They treat you like your the worlds biggest moron. Of course their can't possibly be something wrong with one of their products. It has to be you.

Sorry about this being so long but can anybody else give me an idea what to do. My next step is to just go with a different cam maker at the moment.