If only the 1 - 2 shift is hard but not the subsequent shifts, then the
vacuum modulator is working as it should.

Some M-B shifts ARE hardER than those found in modern cars that have
electronic engine and transmission controls which briefly retard the
ignition to depower their engines at the instant of a shift. We're
spoiled by these new fangled cars!

Back to your car. It could be that the transmission is not shifting WHEN
it should be; that can be easily checked, even by a non mechanic.

At the top back of the engine, on its right side is a rod or cable that
links the throttle linkage on the top of the motor to the right side of
the transmission. Its purpose is to "tell" the transmission what's going
on with the throttle so the transmission will "know" when to shift. It
could be that this rod or cable is not correctly adjusted. To check its
adjustment (motor shut off) one pries the ball at the upper end of the
rod or cable off its pin on the throttle linkage. Gently pull the rod or
cable as far as it will go (gently) and allow it to return to is own
natural position. It's correctly adjusted if you can reattach its ball
to its pin WITHOUT pushing or pulling the rod or cable. If you can't
reattach it then lengthen or shorten the rod or cable so it CAN be
reattached as described. Any adjustment should be minor but may improve
the 1 - 2 shift.