Sierra6.0

So I rebuilt the lq9 that is in my scsb nothing crazy was done I used the same cam 224/224 114 lsa and chromoly pushrods and other basic bolt ons headers cold air intake and that's about it I bought the truck and dropped a valve in #2 after driving it for around 4K miles and I bought another block because that one was scored up pretty good and didn't want to bore over so I saw good reviews for DNJ rebuild kits so I replaced the pistons and rings , bearings and all gaskets and o rings I used the same rods and crank from the block I bought all seemed well I replaced the brand new oil pump with the kit when I was driving and dropped all oil pressure caught it in time **** it off got it pulled home that was after 400 on the rebuild I used the old oil pump to make sure that was the cause no problems with oil pressure after and driving home from work it broke down on my piston cracked in #6 after 4K miles I was shocked I did all torque turn procedures I didn't understand why I had been driving conservatively and only spun it up fast a couple times , I didn't have it tuned previous owner said he had the computer replaced and programmed for the cam at a nearby shop but I'm thinking this isn't the case I think ignition timing could've been off and was pre igniting any thoughts?

KCS

iTrader: (20)

Post pics of the damage.

Sierra6.0

Haven't torn it down yet just put a bore scope in and saw it's cracked most of the way across I got underneath and it was spinning freely from the flywheel but would stop at one point the more I moved it back and forth the less travel I got now there isn't any movement hardly

KCS

iTrader: (20)

Can't really do a proper autopsy if you don't open it up