shifting problem

i have a 1977 c-4 after inital start up and drive off it takes forever for the trans to shit the first time sometimes up to 40 mph.after that it will shift normally---any ideas???

Changing the fluid and filter is always a good place to start if it hasnt been changed in a while. Also check the vacumn modulator and the line going to it. Pull the hose off at the trans, if theres evidence of fluid in the line, the modulator is bad, it helps control the shift points. Good luck.
Later
Ed