Sorry, I had my user profile set up so it wasn't showing the signatures. This is because I was having a hard time with the speed of my lousy dial-up connection, and the less info my computer had to process, the faster it was (although still very slow). It's been acting better lately, so I turned "show signature"
back on for now.
I'd start with checking the port on the brake booster line where the smaller vacuum line connects for the modulator. These tend to plug up over time and can reduce the "amount" of vacuum available to the vacuum modulator on the transmission, which on this model can affect the shift quality (soft/firm) and I also believe on an 83 it can affect the shift point as well.
If you don't seem to have any flow out of this small side port on the brake booster line, as a test you can try clearing out the port with a small wire, such as a torch tip cleaner wire. But the size of this orifice is considered an important dimension, so i would normally recommend replacing the booster line itself if this seems to be helping the shifting, they cost in the $25-40 range and are EASY to replace.
You can also check the modulator itself with a MityVac to make sure it's holding vacuum.