Look at the left side of the intake manifold. You'll see a large sensor, that's the boost limit sensor. Immediately below that you'll find a hose fitting. Undo the hose, and screw the fitting out of the manifold. I bet you'll find it plugged with tarry soot. Just clean it up and you should be fine.

If the transmission shifts through the gears at the correct RPM / road speed relative to throttle opening, you have other issues.

If the engine is down on power, the shifting will be late and hard due to the throttle being open farther and trans expecting more power.

Did you have someone push the throttle ( engine off ) while you check that the linkage travels full stroke under the hood?

Don't just randomly condemn parts, troubleshoot.

The 1985 300D uses a 722.4 transmission, or at least CA models do. I agree that it doesn't sound like the transmission is your problem.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

You said it shifts fine, but goes slow.

Could it be the fuel tank filter that's clogged up? Over the years it gets very occluded with that algae and the tank and filter need to be cleaned. It will severely limit how fast the car will go if it's clogged, especially uphill when the engine really needs the fuel.