I think there is more going on with a '76 safety switch than I can figure out right now. The safety switch appears to be providing a ground for the coil of the fuel pump relay. Full throttle plate deflection should be removing the ground and shutting off the relay. Why it would behave as you say with key on, engine off is a stumper to me, but I'm sure it behaves as you say.
On Gunter's car, I am a little perplexed also, maybe Steve can help. What i would ask Gunter to check next is this:
Check for good battery power on socket 2, should have battery voltage all the time, not just with the key on, this is constant battery power. Next, with the key OFF, make sure there is "0" volts at socket #1. Then also check for "0" volts at socket 1 with the key ON. The only power getting onto the circuit that is on that socket 1 should be coming from the relay. If there is power getting there some other way, this is probably the root cause of the problem.
The components on this circuit are the fuel pump itself, plus the warm up enrichment compensator anf the frequency valve. The frequency valve is probably the main suspect, or actually the engine control module, which provides the ground for the frequency valve. It could be that the engine control module is bad and feeding back power through the frequency valve.
Maybe Steve knows, but if this is the case, as a test I believe you could operate the engine without the control module at all in fixed operating mode, just to see if the fuel pump then requires either the relay or a jumper cable to be in place.
Alternatively, there could be a short to power in the socket #1 circuit which may mess things up a bit also.