Have a 1970 mustang automatic won`t shift from park

Could be the park neutral switch. It senses that foot is on the brake pedal so shifter can be moved.

Could be the brake switch, or the shift lock solenoid.
Both components would need to be tested. And you may be able to shift out of park if you try before you turn the key switch to the on position.

Rock the car back and forth to remove parking pin from park .

Automatic or stick ?
If its an automatic, it could be something in the torque converter or transmission. The torque converter clutch could be stuck.

http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?fromSearchPage=true&pageId=0900c15280251027&partName=Neutral+Safety+and+Back-up+Light+Switches&partId=0900c15280251027 Here you go,try this.

Shift - interlock system problem .
Automatic Transmission Shift Lock Control System
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of PARK position. The system consists of the following components:
• The automatic transmission shift lock control solenoid
• The automatic transmission park lock solenoid
• The automatic transmission shift lock control switch
• The transmission range switch
With the ignition in the ON position, battery voltage is supplied to the automatic transmission shift lock control switch. The circuit continues through the normally-closed switch to the automatic transmission shift lock control solenoid. The transmission range switch provides a path to ground for the automatic transmission shift lock control solenoid in the PARK position. With the automatic transmission shift lock control solenoid energized the shift lever is mechanically locked in the PARK position. When the driver presses the brake pedal, the contacts in the automatic transmission shift lock control switch open, de-energizing the automatic transmission shift lock control solenoid. This allows the shift lever to move from the PARK position. When the shift lever is out of the PARK position the controlled ground contacts in the transmission range switch open. This prevents the automatic transmission shift lock solenoid from energizing when the vehicle is out of the PARK position.

look under car on drivers side of transmission...shift cable may be loose..try to push it in as far as possible.

SOUNDS LIKE SHIFT INTERLOCK SOLENOID IS STUCK. TRY GIVING A FEW HITS AND PUSH ON AND OFF ON BRAKE PEDAL AND SEE IF YOU CAN FREE IT UP. JAR IT AROUND A BIT LIKE TRYING TO WIGGLE IT OUT WHICLE PUSHING ON AND OFF ON BRAKE PEDAL SEVERL TIMES. DO YOU HEAR A CLICK IN CENTER CONSOLE WHEN PUSHING BRAKE?

the C4 automatic transmission is a 3 speed transmission, it shifts by hydraulic pressure controlled by a governor and actuators, I would check your linkage to see that it is hooked up correctly to allow movement through all range selections, disconnect the linkage at the trans and see how many detents you feel while moving the lever back and forth, make sure to chock your vehicle as moving the lever from Park may cause the car to move. Let me know what you find, hope this helps

Check and see if your brake lights are functioning. If they are not then look for fuse #9 to be burned out. The shift interlock on these cars is dependent of brake light circuit and it BOO (brake on/off) switch to release the shift interlock.