1977 Chevrolet Custom Deluxe tranmission fluild

It's either a clogged pickup in the filter assembly or you've got too much fluid in it.

Slide under and take a picture of the transmission oil pan.
Take that picture to your local parts store along with the year, make, and model of the vehicle and the engine size. Preferably the transmission model too, and pick up a filter kit and enough of the right type of transmission fluid to refill the pan. Also get a catch pan if you don't own one and a container to transport the old fluid in safely.
Jack the vehicle up enough to slide under safely.
Loosen the pan, leaving a few bolts in on one side so most of the fluid can drain out without making a mess.
Pull the pan and old filter.
Install the new filter.
Clean the old gasket off the case and pan and reinstall the pan with the new gasket. Don't over-tighten the bolts or the pan will leak.
Take it to a shop that does transmission flushes to flush out the rest of the old fluid. You'll be "wasting" about 4-5 quarts of new fluid, but it's better than buying a new or rebuilt transmission.