To stop fuel flow, you'd need to turn off the ignition.
Some possibilities to consider, though highly unlikely on a stock 1987 G20:
* Aftermarket Installation: Someone might have added a fuel cutoff switch at some point. If this is the case, its location could be anywhere – under the dash, in the engine compartment (near the fuel pump), or even hidden elsewhere. You'd need to check wiring diagrams, look for unusual switches, or consult the van's service history if available.
In short, expect to shut off the fuel supply by turning the ignition key to the "off" position.