I don't think you'll need to make any changes necessarily. A tunes always great to make the best of it. I know on the trucks they get their speed input from a tone ring on the ring gear so it doesn't matter what gear you install, the speedo will still be correct, but I think the cars get it from the output on the trans? If so you'd need a tune to get the speedo working correctly. Now that I think about it, if the speedo is off and the rear wheels don't match the front through the ABS sensors it might give you problems. The Mopar 11 Chargers had that combo factory (5.7/NAG1/3.91's) btw