Can't say from experience but, from what seems to be involved in putting in just about any trans that these cars didn't come from the factory with, I'd imagine a LOT OF HASSLE. You'll probably have to cut & reshape the floorpan & the cross member where the rear of the torsion bars attach. You'll also have to find a way to attach the trans to the engine. Don't know if it can bolt to any bell housing that attaches to the engines that are commonly found in A bodies or not. You'll need to figure out a clutch linkage, easiest way is most likely hydraulic.
Unless somebody is giving you a killer deal on the Viper trans it would probably be a lot less headache & maybe cheaper in the long run to use the new 5 speed overdrive trans from Passon Performance. It's a direct bolt-in replacement for what you have now.