AMG does indeed make TQ monsters. The V12 biturbo makes 740 ft/lbs stock mated to their 7g tronic. The V8 biturbos come with 660 ft/lbs stock in the SL63 and S63 models running on the 7-MCT transmission. Many of these have been modded making well over 700 ft/lbs at the wheels with no issues on the stock transmission. Their upcoming 9 speed is rated for 740 ft/lbs on paper. They do sell their transmissions. Porsche used to use their 5 speed in the 996 and 997.1 turbo before switching to the PDK in the 997.2. The only problem is none of these are DCTs. AMG does have a DCT but they admit it can't handle the TQ of the big turbo motors. None of the DCTs like the Porsches PDK, BMW's, or Audi's DCT can handle much over 500 ft/lbs. There are DCTs that can handle more in exotic cars like the Bugatti Veyron's DCT, but the transmission is big, heavy, and costs more than an entire Viper. Until there's a more mainstream DCT that can handle the extra TQ you might be stuck with a regular automatic. Which might not be so bas autos have come a long way. The ZF 8 speed is highly rated for example.
Eliphaz here. Its interesting that you said they sell their transmissions. Im sure Fiat knows this but I wonder if they would go that route for the Vipers sake?