If memory serves GM did not use an inertia switch for crash shutoff of the fuel pump. They relied on the engine oil pressure to maintain the power to fuel pump. The idea is that, in a crash, if the engine is not running the fuel pump will also not run.
I believe Ford used an inertia switch.
To test the system install a fuel pressure gauge on the injector manifold.
Key on starts a 2 second prime run of the fuel pump.
Engine cranking, oil pressure increases and runs the fuel pump.
Engine running, oil pressure = fuel pressure.
There is a gray wire to the fuel pump. If this wire is broken run a new one.