Well, if the redlines I used are accurate, (6bt@3000/7.3@3500)55mph=47mph.
Someone was selling a gear vendors overdrive in the classified forum a few weeks back, 47/gv@.78=60mph.
If you would rather change rear end gears, a 3.54 would give you (5.13/3.54)55mph=80mph@3000.
A 3.54 would cruise at 55mph@2062rpm, and the Cummins is very happy somewhere near 2000rpm.
All this is based on the accuracy of your stated 55mph top speed and the redlines I used. Good luck with your conversion!