A NP435 is 6.xx in first 3.xx in second 1.34(
) in third and 1 to 1 in fourth. Unless you are hill climbing with a load 3.23s will still get you around town and do great on the highway.My Dad had a 69 F100 with t18 and 240 plus 3.23s and at 75 spun 2300 rpm in fourth. My 350 with 4:10s does 4400 rpm at 85. The thing is I can climb the same hill (4000lbs load) he climbed with 800 lbs of hay with a horse trailer with horses (2100lbs) and we both had to do it in 3rd.
No load 4th Gear no problem for both trucks with radically different rear gears. So do what you want with the gears unless you constantly haul maximum loads.
Fuel economy with a truck 4-speed go low, with a C-4 or 6 keep it in the low to middle 3.