Your IAC (Idle Air Control) is bad. Most likely the plunger device is clogged up. I have no idea where it is on a 302 V-8, but on my V-6 Explorer, it's right by the throttle body (It will be close to it) and is cylindrical shaped. Two bolts and an electrical connector is all that holds it on. Spray some cleaner into it. If this doesn't help, the part is around $70.
It's incredibly easy to fix. The way I knew mine was bad was that just like you, so long as I would keep my foot on the air pedal, the motor would run, but as soon as I took it off, the motor died. The IAC makes it so you should never have to put your foot on the pedal while starting the car like you would on an old carbeurated machine. If you do have to do this, your IAC is bad.
