Virgis, going over the wiring diagrams again, it appears I gave you some incorrect info. That injector fuse also powers a circuit in the ignition module, so interrupting that will in all likelihood prevent the ignition from working. Pulling the fuel pump relay will disable that circuit, but GM has a backup power path to the fuel pump through the oil pressure switch. Once the engine starts and develops oil pressure, one side of that switch closes and provides an alternative path for power to the fuel pump should the relay fail. The only other route is to disconnect the wiring right at the fuel pump connector itself, and I am not sure if that can be accessed without dropping the fuel tank.
Again, I am not sure how the LPG installation was made, but we had some Dae Woo forklifts at work what were solely propane powered. The fuel lock ( solenoid that turns on and off the propane) was powered 2 ways. First directly off the starter to turn on fuel during cranking, then a oil pressure powered switch to keep the propane flowing as the engine was running. They may have tied into that circuit on your van; it is a gray coloured wire.
The forklifts also had propane filters in the circuit, not positive but I seem to think they were on the low pressure circuit( been over 10 years since I worked on one). They were prone to fouling up and getting blocked, causing problems
Jeff