if it has fule an fire does it have compreson maybe you broke your timeing chain but not lightly. have you cheked your plugs for fule maybe it is geting to much.trie puting it in clear flood mod with your key in the off postion hold the gas petel ...
From what i now of 5.3 ls engine that ecm keeps the timing advanced from the knock sens I dont think there is much or any change to be made. The ecm does a good job.
how does your 383 perform? I was just planning to go with forged pistions an arp rod bolts an thats all i was going to do to the lower end not planing on seeing to many rpm