6-speed transmissions for US models?

I just picked up a 1991 V8Q 5-speed with transplanted 4.2 liter engine. There are 7 of them in the US.

I'd like to make it even more rare by putting in a 6-speed.

Does anyone know the transmission code for a 6-speed? Are there any in the US? Would any later model 6-speeds work ? S4? Allroad? A6 2.7T?

I bet the cost would be prohibitive, but I'm just curious.