3 on the tree would be great for nostalgia, but not so much for a DD. I run a 4 speed NP435 with a granny low. Once you get used to not having overdrive it really isn't terrible and I drive it for an hour at a time on the highway on my way back and forth to work. Check out this calculator. You can plug in your ratios and tire size to figure out what rpm you will be running at, just to make sure you don't push the engine too hard. If you click on the frame that says "calc" on the bottom it will come up with an empty table that you can fill in with a possible 6 gears. The numbers shown in this one are from my truck.
http://xse.com/leres/bin/gearratio?t...le=60&wheel=15