It won't fit. No, seriously, that's the reason. We talked with SRT powertrain director Chris Cowland this past week at length about the topic and it's simply not going to happen. You can read about the several critical reasons why over on Motor Authority at the link below.
We will say this interesting tidbit came from that conversation with Mr. Cowland, "Somebody will do it, it won't be us." The reason we say interestingly is because if you read why the Hellcat won't fit into the Viper, it's not a simple fix. Which means someone might have to do some heavy lifting to make it happen.
While the Viper won't get the Hellcat V-8, the Hellcat does use the Viper's manual transmission. Which then begs the question as to whether the Viper would ever get the Hellcat V-8's eight-speed automatic transmission.
In the mean time, we are all obviously expecting the Hellcat V-8 to go into the Charger SRT sooner than later. Because only us Americans would make a 707-horsepower four-door family sedan. 'Murica, $%$* yeah!