Transmission Shifting

Never hurts to change the transmission fluid & filter.

These boxes DO shift harder than we're used to, contemporary cars have
smoother shifts from their electronically controlled boxes.

If ALL shifts were neck snappers then the vacuum powered shift
modulation isn't working but you wrote the higher shifts were OK. So
that's not the problem here.

You can check if the transmission's shift point is correctly adjusted.
It's easy.

On the top rear of the engine, on its right side, you'll find a rod or
cable that goes down to the transmission's right side.
Use a flat blade screw driver to pry the ball socket on the rod's top
end off its pin on the throttle linkage.
Gently pull the rod and allow it to return to where ever it wants to be
(its "natural position").
The rod's length is correct if, but only if, you can place the ball back
onto its pin WITHOUT pulling or pushing the rod.

If pushing or pulling is needed, the rod's length ought to be changed.
The rod's ball is secured by a lock nut, loosen it and turn the ball to
lengthen or shorten the rod, snug its lock nut. If your car has a cable
turn the cable's mounting nut that's on the top end of its outer case.

When this adjustment is correct snap the ball onto the pin (first give
it a bit of ATF, oil or grease).

Now the transmission will correctly "know" the throttle position and
shift accordingly, but firmly.