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The longer answer is, given enough time, money and skill, anything is possible. But the cost would far exceed the cost of buying a brand new (or drivable, used, low mileage used) C7 and driving that.
You'd have to fabricate a rear frame section and rear suspension that matches the C7, and graft that onto the C3 frame. You'd also have to install a modern LS-x engine with electronic throttle (DBW throttle) to work with the A8 automatic. The A8 automatic transmission was only used with DBW throttle configurations, and the transmission requires that feature to operate correctly.
Lol. It was a dream. I've got a 383 stroker. I've got a 350 trans, and its built with heavy duty cluches. And a shift kit, and a 3500 stall converter. Maybe I don't even need it. I was just thinking about getting the rpms down. 3000 rpms @70 mph. Whoosh. 6 miles to the gallon.
Well, you'd want the version of an 8-speed that was mounted behind the engine with a driveshaft, not at the end of torque tube with the differential directly mounted. So, truck one or maybe from a Camaro.
A controller for stand-alone use still seems to be an issue. A 6-speed would be much easier. Others have already done it. Apparently, it works very well. Much better than 4-speed auto swaps.