I'm hooking up a HEI in a car that did not have one originaly. From what I understand I need to run the power wire to the fuse box, and to a source that has power at ignition, or is it called accesories, it's the fist notch when turning the key clockwise, not actually trying to start the car. There is a place on the fuse box that has nothing going to it, and has power at ignition, but it is non fused. Is this an ok place to run it or should I tap into another supply that is fused, or if it needs to be fused can I just add an inline one.

You want to use the one that is "on" in the "run" position, not ACC. This would be when you turn to start and release the key. If you use the ACC position, power would be on to the ignition while you listened to the radio, not good. I ran a 12ga wire directly to the dist and remember NO RESISTOR. (The spot I chose on my fusebox was hot in either START or ON position. Some of the older cars use a wire from the solenoid to power the ignition while cranking.)

But wouldn't the power stop when the key sprung back after start up attempt?

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Read it again.... You want power when the switch is in the ON position. When you let go of the key while cranking, it goes to the ON position. Power while cranking is normall y supplied by a wire from the solenoid on most cars. On my car with HEI (a '33 chevy street rod - all Chevelle running gear) I have a van fuse block that has a position with +12VDC during ON and START (cranking) this is where I added the circuit to the HEI.