I think what it actually is, is just the 500E or E500 as it became from 93' onwards. It is basically a modified 300E. Porsche engineers helped design the mods. It was partially assembled and modified a little (wheel arches and inner wings) at Porsche- Zuffenhausen. But I doubt you could get 6 litres out of it because, to get the 5 litre (from the 500 SL) into the smaller 124 engine bay they had to lower the blocks deck height, shorten the pistons and change the conrods. It is a limited edition car however- 10,000 were made between 91' and 94'. Hey, Did you play guitar with the Police?
Actually, most of the assembly process and modifications were carried out at the Porsche factory. Parts and paintjobs were provided my Mercedes.
It's very possible to increase displacement to 6.0L. Off the top of my head, AMG, Brabus, Carlsson, Hagmann, and RENNTech all offer 6.0L conversions (I can't confirm whether Lorinser ever had a similar program). These engines typically put out ~400 hp and ~420 lb/ft (varies from tuner to tuner). A fellow over at MercedesShop has the RENNTech 6.0L (full track setup as well, including 14" front brakes) and catches many Porsche/Vette/Viper drivers by surprise on track days.
Given a choice (and a suitcase of money), I think I'd opt for the Brabus conversion, as I believe it yields the most hp/torque, yet is less expensive than the RENNTech. Also, Brabus cars are damn cool. [;)] They even built an E500 V12 (can't recall displacement), but that's bordering on lunacy. Not that I wouldn't jump at the opportunity to own one!
Anyway, back to the original topic, I don't think the car on ebay is a real AMG 6.0L. You'd think there'd be at least one photo of the engine bay confirming the claimed modifications. Probably just a "run-of-the-mill" 500E.