Well you didn't say what year it was, keeping that in mind I will try to respond with the info you did give. Generally the Chevy G20 vans have a coil mounted in the distributor itself, so having spark coming from the coil would mean it is not flowing through the rotor to the plug wires.... not sounding right to me either. Possibly just a bad Distributor cap unless you have already replaced it. So I would have you look at the Ignition module first and the pick up coil second.. not mush else it could be (ruling out a broken timing chain)
Troubleshooting: