1979 Chevy Caprice Wont shift

This car has a 350 transmission and all of a sudden it stopped shifting. It will not come out of first gear. As you accelerate and then lift off the throttle it seems to freewheel like it's coasting. I thought it might be the modulator and I changed it but that didn't do anything. The trans is full of oil and the vacuum to the modulator is good. I checked the governor to see if it was stuck and it is ok. I have not checked pressures but that will be my next step however I don't know what the pressures should be. Can guide me to the next step?