Switched 12v source

Some great information in this thread.
I just went through a similar dilemma with my 71 Duster - 440 in it.

I had no factory wiring as this was a former drag car I was bringing it back to Street. It had been previously wired with all toggle switches and a push button for a the starter. I ripped all that out and wanted to start and turn off the car with the Key and eliminate all toggles.

Also important to me to make sure a Neutral Safety Switch was working properly. I wanted to use a relay for the Run Circuit as well since I am adding a Holley Super Sniper in the future. For ignition I am Using an MSD 6AL-2.
I used an MSD 7564 Solid State Relay Pack (4 Channels) to run Starter Solenoid, Run, Fuel Pump and Fan.

Coming off the Ignition Switch,
I ran ACC to a fused bus for all ACC circuits I would want.
IGN 1 and IGN 2 I ran to the +12 Input for the Run Circuit (Another fused Bus on the output from the relay).
Ran from the Run Circuit Bus to the +12 input on the relay for the Fuel Pump. (Fuel Pump Only Runs in the Run Position and turns off when key is off)
Ran a -12 (Ground) input from the Temp switch to the Fan Relay.
The challenge came in with the Start Circuit since I wanted a NSS working properly. The MSD Relay Pack is a solid state relay and each channel only used a single input (can be either +12 or -12 but not both). Normally on a NSS I would just break the negative side if in gear and use the switch in the trans but that was not an option with this relay pack. So I ran the Start wire from the Ign Switch through the switch in the aftermarket shifter in the car (B&M Pro Stealth) to the +12 side of the Starter Solenoid relay.

Neutral Safety Switch works and I have a clean Run Bus for when I add Holley Sniper later. No Toggles and key turn everything on and off (Except fan which will run even if car is off if needed)

There are probably better ways to do this but this is what worked for me.