Is your wiring otherwise stock? Pulling power from the starter relay seems a bit of a hassle, if you still happen to have the Red w/green stripe wire running behind the engine.
Then again, it might make things simpler once you install a relay.
The two sources that have power only when the key is in the ON or START positions are the Green w/red wire at the voltage regulator, and the Red w/green wire at the ignition coil. Depending on what year your Bronco is, there may be only one of each, or there could be more than one.
But there is only one source from the ignition switch and both wires attach there.
The Brown wire from the "I" post of the starter relay/solenoid is directly connected to the Red w/green wire, which is why it has power with the key ON.
If the stock wiring, then the Red w/green wire is a resistor wire with lower voltage. However you can still use it to trigger the previously discussed relay and then that relay can power the ignition coil and distributor.
With a new relay you could still use the starter relay's "I" power with the Brown wire for your trigger source and then it's easy to connect the relay to the battery. Then use the existing wire that was previously powering your HEI distributor to power both the coil and distributor's positive wires.
So you have plenty of options and can run the wiring any way you want, as long as you use one of those as your source.
The reason for this particular wiring as your source is so that you can avoid the problem you mentioned initially, where if you leave the key on you might overheat some of your components. With the Green w/red and Red w/green wires, you can leave the key in the ACC position as long as you want, listening to the radio or working on other electrical stuff, without any power going to the ignition components.
Only when the key is in the ON position will you have to be concerned with how long you leave it that way.
Paul