In my 97 tahoe swap I used the wire that went to the 350 distributor that has 12V when the key is in the on and start position.
I swapped chevy oil and water temp sensors in so that my factory gauges worked. I also used a gm crankshaft position sensor for the tach.You need 4 bumps on the vibration damper to trigger that sensor.
You just want to have a 12 volt source that is on when the ignition is on. The VE pump solenoid doesn't have a huge current draw like the P pump so a relay probably isn't necessary but won't hurt if you want to do that. Another option is convert to manual operation with a pull cable and forget the electrical.
I had considered the manual shut off since everything else drivetrain-related is now converted to manual control. I decided though to repurpose the factory fuse and 12v wiring for the old ignition coil and should be good to go.
Next, I need to get this alternator tach sorted and I think I’ll finally have all the electrical finished.