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When you replace the lifters, you should have soaked them in a "assembly lubricant" sold by GM. It costs about $8.00 and change. I don't know if this would have necessarily resolved your problem however.

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find the recommended oil weight for the truck. I do believe that the poper oil weight is 10/30 or 5/30 anything more will make you sound like a singer sewing machine

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GM gets many complaints about noisy lifters. A rep from GM Performance Parts division tells mechanics to set the hyd. roller lifters similar to the way solid lifter racing cams are set. By this I mean with the engine off and warmed up, go to one set of valves, rotate the eng by hand to place the intake valve on the heel or base circle of the cam. This is done by observing the exhaust valve when it is half open stop and adjust the lash on the intake rocker arm to zero and then turn down preload to 1/4 to 1/2 turn, not the one full turn you find in manuals. I use 1/4 turn. Now to do the exhaust valve lash, rotate the engine until the intake valve is going approx. half closed, then stop and adjust the exhaust valve rocker to zero plus 1/4 to 1/2 turn. Continue to adjust each set of valves in the firing order. The first time to try this it will be tedious and late model eng. are very hard to wrench over. This keeps the lash from being incorrectly set on the side of the cam lobe. A little bit easier way would be to set intake and exhaust together with the cylinder on TDC compression stroke. With both valves closed, set the preload. This is what has worked for me. Also use the correct oil, not the 10W-40 of old.

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Sounds like piston slap to me. It's common on Chevy vortec because of the short piston skirts.

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Could be from a leak in the notorious "intake gasket" made by Chevrolet/GM during this year. The gasket may have leaked, causing coolant to enter the oil circulation..... Check your coolant level.