i cant answer this, but why not go 6.0? more hp for a big truck...just a thought.

i think you can use the spacer/flexplate from the 6.0 on the 5.3 to be able to use the 80e. i dont know for sure if they swapped converter bolt patterns but id check into it. or just have a custom stall made to the 5.3 flex plate and your 99 classic 80e.

so can we get some pics or what?
what happened to the 5.7?