the W126 largest engine option was the 5.6litre engine in the 560SEL, appart from that there was an AMG 6.0 but very few were sold.
Some speciality companies made a 1000SEL but that was just a 560 with silly options like rear tray tables etc.. tailored to the arab market.
BTW you cannot bore the 560 engine to 8 litres. max done so far by brabus was 6.2 litres.