To me it looks like a timing issue. The heads had dual valve springs on them with titanium retainers valve float wasnt an issue IMO on that setup since the bottom end with stock rod bolts would let go before there would be any valve float unless he was running a solid roller cam or something in the 650 lift or higher range. Also you need to remember the intake valve is closed for a split second after combustion where the exhaust valve is open and the extremely hot gases are flowing past the exhaust valve super heating it. I agree with NITROUS that if anything it looks like it was running rich. When I sprayed my 5.3 it was a running a little rich and when I pulled it for the 418 the piston tops had alot of carbon buildup on them and werent melted or distorted at all. To me he either spun it up way too high and a rod bolt let go causing all the carnage which owuld thorw the balance of the assembly out of wack and cause the valves to hit since he was running a large cam or the damage was already done from an earlier pass and the piston was toast and let loose on that run causing the damage. What kind of t-chain was he running? Is this an LS1 motor or LS2 Im not up to speed on the differences on the LS2 but form what I understand they are minimal. I dont understand #6 letting go usually its number 7 that goes before the rest. Allen did you check the tune to see what he shifted at, is it an auto or man. If its an auto did he have a shift kit in it so he could shift it at whatever? What did the customer describe what happened? Sorry for all the ?s but they would need to be answered to come to a conclusion as to what happened.