A quick follow up. Shifting is much improved with a fresh fill of Syncromesh fluid. I no longer have to pause for a second between shifts which makes keeping up with traffic much easier. Also, I think the overdrive may be working but not the way the owner's manual says it should. The owner's manual states that "pulling this button out to the limit of its causes the overdrive unit to be inoperative. Do not pull this button out when going more than 35 MPH. However it may be pushed in at any speed while the clutch is disengaged."
Mine seems to engage overdrive when the knob is pulled out, not pushed in. Also, while the car was in motion (well above 35 MPH), I depressed the clutch and tried to push the knob in but it didn't want to move. When I attempted to force it, I heard a bit of gear clashing and quickly backed off. I need to get back under the car and see if maybe the control lever was flipped so that it's 180 degrees out from where it would have been originally.