I just did a 07 DTS the other day that was overheating. I thought it was the headgaskets but ended up being the coolant crossover gaskets leaking when it was cold. Luckily I pressure tested it the night before I tore the engine out to do the headgaskets! Came in to a big puddle of antifreeze on my floor the next morning. They would have been changed while doing the headgaskets and it was under warranty, but I'm pretty glad I found a definite problem and know it'* repaired for sure now. BTW it was NOT a fun job!
2000 Silverado RCSB 5.7L LS6 - 5 speed (Ex 4.3L V6)
T4 Turbo/Intercooler, Lunati cam - 220/224 .575/.575 112lsa, 317 heads, Edelbrock shorty headers, Magnaflow cat-back, CAI, Nelson Performance tune.