I haven't messed with old chevys in a while but it could be something as simple as a bad vacuum modulator or a crack in the little rubber hose going onto the modulater making it hold the shift, seemed always to be the problem whenever one of my old camaros had the same symptoms.

You can also get an adjustable modulator and change the pressure it shifts at I believe. Like I said, I got out of old chevy's about 9 years ago.