I had one car that had a worn out clutch plate.

It would shudder when accelerating hard. However I did not drive it much until after the clutch was repaired.

However my perception was that if I did not feel the clutch slipping I did not feel a loss in speed.

So I can not say how much a bad clutch is affecting your top speed. But I do not think it is effecting your top speed that much.

You said the original wheels were found. Do you know if the tires on the truck now have a correct diameter. Smaller replacement tire will lower your top speed and throw off your speedometer.

A tired old engine with even low compression will run smooth though will not have the power it once had.

Through the years was the wrong carb put on or smaller jets installed.

Verify your speedometer with a GPS.

Find out what your rear end ratio is. It could of been changed.