Hissing sound from under dash

If you have an automatic parking brake release check the area around the top of the parking brake pedal and the bottom of the steering column, near the floor, for a disconnected vacuum hose. Also see if the hissing stops with the transmission in neutral. If so that's what it is. Automatic parking brake release systems worked by vacuum in those days and were hooked up to the neutral safety switch. When in gear vacuum is sent from the neutral safety switch to an actuator that's hooked up to the parking brake pedal. The parking brake pedal must lock down in park and neutral so no vacuum is sent to the actuator when the shifter is in those two positions. Hope that helps! Jim