When I got the 83 D150 from my Dad back around 2010, level ground highway pedal to metal top speed was about 70 mph. This is a short bed D150, slant, factory 2bb, a833 od transmission with 3:23 rear gears. Any kind of hill at all and one would need to down shift to third to maintain 70.
The truck had been well maintained, had about 100,000 miles on it. Those trucks are absolute bricks aerodynamically, factor in the lean burn ignition, the ridiculously retarded stock cam shaft timing and the D150's with slant motors of that era are RPM and highway challenged.
Today with the compression boost, hei ignition, a small but normally configured camshaft and a free flowing exhaust the truck will easily do 90 mph.

To the originator of this thread,
1) do an evaluation of the engine and the engine tune: when was the last time it got new plugs, spark plug wires, distributor cap, if it is still running the lean burn, consider an HEI, get a distributor with a vacuum advance and do a re curve on it for a heavy vehicle, look around for a 2 bbl carb and intake, do a compression test to understand how healthy the motor is, monitor oil consumption and so forth.
2) if it's in the budget consider an engine rebuild, get the compression up to 8.5:1, you can stay with the stock valves but get the best multiangle valve seat you can afford, replace the timing chain, install a smallish cam that is not ground retarded (by smallish, a cam less than 210 duration at .050) and install it 102 to 105 ICL, a cold air intake helps.

It will be a journey but there are folks here at .org that can offer sound advice.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8