".....Since the cutoff valves use a magnetic solenoid I would expect they require some considerable current flow so my question is what is the best circuit to tap into for the ignition-on valve flow?........"

Actually as I recall the current draw is pretty minimal (not sure how much though).

"......Very simple. Just wire it to the accessory fuse that's hot in the run position only......"

I haven't gotten past the point of just building the harness yet but Denny prett much hit it on the head as far as what I'll do when I get to that point. I'll be looking at whatever is hot in the run position.

Traditionally this era of Dodge has some issues with fuze box terminals heating/melting on a couple of the higher draw circuits (the AC for example). On those I will be using relays. Depending on how the fuze box is wired, the wiper circuit, or possibly the AC circuit (before the relay) come to mind.