That could be a few different issues, if shifting late like you said. I would do a tranny flush and screen change. Also change spark plugs and wires. Carburetor could cause sluggish reaction. What motor do you have?

 

I would guess it probably the carberator needs some attention.Odds are it needs a rebuild which is nothing unusual for a 38 yo part. You should definitely put in a fuel filter in the carb before rebuilding. To check for a it leaks on the carberator, try spraying carb cleaner around and on the carb. If the idle changes you will need a rebuild.Also depending on the transmission , if its a. 4 speed automatic the shift points are also controlled by the adjustment of the detent (kickdown) cable located on the carberator. Changing the transmission fluid won't hurt either, probably cost ya about $30 if you do it yourself. Good luck