The governor is located under a round metal cover on the side of the case. Some of them have a spring wire bail securing the cover, while others just drive in and hold like a hubcap. Remove the cover, using a hammer and punch, and pull the governor assembly out. See if the weights are free to move, and check the gear teeth to be sure they're not worn down in the middle- - - -known as "apple core" wear. If the governor is in good shape, check the vacuum line between the intake manifold and the governor for splits, leaks, or holes. Vacuum leaks will cause shifts to happen at very high RPM- - - -a stuck governor or an apple-cored governor drive gear will keep the trans from shifting at all. Debris in the valve body can restrict the movement of the valves and cause shifting problems, but if that's the case you need to find another "rebuilder". Some guys don't disassemble and clean the valve body (Mr. Bad-Wrench!)
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!