Swap issues...

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If this is in the wrong place, please advise.

Back story and PO new very little or played it that way:
New to me 2001 Sierra 2500. "G" engine code = 8.1L
It's no longer a big block truck. Not sure which series it is although I did see the 243 head casting on both heads. PO said 6.0L only because the airbox has that sticker. Have not seen the casting numbers on the block as of yet.

My issue:
How do I know it has the correct ecu for this application?
What am I going to be looking for to verify this?
How compatible is the wiring from the 8.1L to the ???
Vehicle runs and drives. Check engine light inop. ABS light on hard. At least 2 dead cylinders, drivers side. Appears to have water intrusion. I have not found any amateur wiring repairs/splices/large balls of black tape...yet. Although the cateye conversion needs help LOL


About me:
Very tenured mechanic with decades of experience. However, I stepped out of the automotive industry before the early LS series of engines were out of manufactures warranty. I have never worked on an LS based engine so all of this is going to be new to me.
Please understand that I'm not out of touch. I just finished building a 190mph ZX11 and I'm still in process of building a GS710 with GSXR suspension. I'm also a parts specialist with MWM and MAN engines. I sell $1500 spark plugs :-)

[unfocused]

Further update:

I removed the P/S and Alt bracket and discovered a 6.0L.
Also discovered 3 dead cylinders, drivers side. Looks like water ingestion.
Complete valve job in my future. Off to spend money...