I just went through the same thing with my 2005 LLY about 6 months ago. It started out as coolant loss over time then after a long trip just getting home, the next morning I drove a few miles, stopped and there was coolant all over the drive way. Checking the engine, the hoses were rock hard and a couple more miles the temperature went sky high. Having remembered a guy trying to buy my truck a while back that if he had it he would go ahead and put in ARP Head bolts and Mahle gaskets and it would be a million mile engine. That's what I did and it will add $1,500 to $2,000 to the cost but, that far in, why skimp? Since then, the truck does show some age related maintenance issues from time to time but it was the right thing to do. I just brought home a 2021 Sierra Denali Black Diamond so I now have to decide what to do with the 05.