Just sell the slushbox and buy one with a real transmission.
Well, the problem is that there is literally no other car like it in the world... in my price range. :) All the other similar cars differ in some major respect. The successors, besides being orders of magnitude more expensive, are also hundreds of pounds heavier. The cost-viable competition isn't made out of Aluminum, either, with all the corrosion resistance that implies.
Anyway, Audi six-speed swaps are pretty well-solved problems with all-factory parts, and putting one into the D2 A8 is no exception. The only part that's hard to come by, and thus commonly worked around, is the right front axle. I think it's the right, anyway. If you can't find a rare euro axle, you need a spacer. It's a pretty simple thing, though. You can also theoretically get a custom axle shaft made up from the old one, but I don't know if anyone has actually gone that route. I forget if you need a totally new pedal box, or just to add the clutch pedal. You don't need a new ECU, though it can take a reflash. That's a a solved problem for Bosch ME7.