The vacuum modulator is on the drivers side of the transmission. It can be one of two versions. The early version has a black rubber cap that cracks and changes vacuum signal, Just replace the cap and roadtest for shift ouality. If the shift is still firm remove the cap and turn the adjuster counter clockwise in small increments until you get the shift you want.
The second version has a white plastic cap and has limited range of adjustability. Turn the plastic cap counter clockwise until you get the shift you want.
Good luck
Mike