I'm pretty sure the GVWR and GCWR are primarily based on suspension, frame, rear end (type not ratio), etc. I tried to look up the ratings for a 2000 R/T, which has the 5.9, but it was a two wheel drive, so I don't think that would be useful. I did note that a standard cab 2WD had the same ratings regardless of the engine, 2.4L, 3.9L, or 4.7L. And the standard cab 4WD had the same ratings regardless of the engine. So I'm thinking your GVWR is still 5400, your payload is still 1490, and your CGWR (also called towing capacity) is still 6400.