Another baby step, really just poking at this project for lack of time to do more. But, progress is progress...
I had a good look at Henry's wiring. It looks like a mess, but he used a lot of terminal blocks and labeled all his wires... This is by far the best documented custom wiring I've ever inherited.
The thing I needed to change here, was the fuel pump wiring. This truck has a TBI, which means the injectors need to be shut off as well as the fuel pump. Henry had the fuel pump on a switch, which was on battery voltage - meaning the relay would stay on all the time if you forgot the switch. The injectors were on a separate switch which is also easy to forget about - if you leave it on unintentionally, it will run "really good" on woodgas. If you leave it off unintentionally, it won't run on gasoline. For my purposes, it needs to go on and off with the fuel pump. The fuel pump needs to go off with the ignition. Easiest way to do this is by using the fuel pump power to run the relay coil - it already comes on and off when it's supposed to, and runs in the crank position (unlike accessory power).
Taking a closer look at Henry's fuel pump switch, it looks like he lifted the ground wire on the rollover switch or something like that... not the power to the fuel pump relay. Because of this, I didn't have 12v+ fuel pump power anywhere near the relay. Instead of messing with that, I just ran a new pair of wires to the fuel pump relay fuse, located on the passenger side firewall. Bingo! I now have control of 12v+ to the pump, which is also a 12v signal telling me when the fuel pump should be on.
For the twin injectors, Henry had already run them to a double pole switch. All I did was substitute a double pole relay, which is controlled from the same place as the fuel pump switch, the 12v feed to the fuel pump. Now when I switch on the fuel pump the injectors get power, and when I switch it off the relay opens and they are instantly shut off. And with the key off, no relays are left engaged, regardless of fuel switch position.
I'm aware there is a use case for leaving the injectors on and pump off, for "trickling in" some gasoline for a mild hybrid mode. For now it's not what I want. If I do ever want it, one more relay on a switch will allow me to manually unlink them and power up the injectors without the pump.