Ok - so it's always the simple ****.

Truck was running better but still running rough and low idle - would sometimes choke out and die.
Was thinking maybe spark plug wires
Then it donned on me that I was using the original 4.8 truck's MAF sensor. ooops.
So I swapped out the one from the donor 5.3 truck and vrooooom. Running tip top

So yeah - no more engine work for a bit - now just replacing fenders and doors. Needs a stereo....
I'll continue the saga in my Project Purple Frost build thread.