Some criticism:
- Finding a decent 8.1 Burb is going to be a hard find and you will need to be prepared to travel to look at one. Will most likely perform worse than the 6.4 Hemi and you mention a 5th wheel could be in your future so that won't happen behind a Burb.
- The Hemi's still appear to have lifter failures and eat the cam lobes and from what I hear the 6.4 may have a slightly higher failure rate over the 5.7. I would avoid prolonged idle time. Stay away from the 6.4 with the 68 transmission.
Good luck and report what you landed with.