Just some random thoughts: The "hiss" sound...do you have vacuum wipers? If so remove the line from the manifold and cap it. One thing out of the way. Is your fuel filter clear so you can see into it? If it is and it is clean, then you might have a blockage at the tank. Not too hard to pull the flex line from the tank, watch out fuel will come out. If it does not, then you are empty, or, plugged. Start with the simple stuff. Check one thing at a time. I have had the fuel line plug. Stops, try's to go and is done. I have had the fuel pump fail. No gas no go. That can be checked at the filter if it is between the pump and carb. just take the line off. Put a can under it and crank a bit. Fuel in the can? Pump is fine. No fuel? Pump is bad. GMC had a positive ground. Chevy did not. Pop the cap off the dizzy. Do the points open and close when you turn the motor by hand? Good. They don't? Reset the gap. Now for some old school tricks. Use a match book cover. Put it between the points. Turn on the key and measure the volts at the + side of the coil. 6 volts if the truck is still 6 volts. Around 9 if it has been converted to 12 volts. turn the key off. If you don't have 6 or 9 volts you have a bad wire to the coil(6 volt) or a bad ballast resistor(12 volt). Got volts? With the key off turn the motor with the match book cover out. Let the points close. Put the match book cover back in. Turn the key on. Put the volt meter back on the coil(+) and pull the match book cover out. Volts should drop to 0. Volts stay? Bad points. Volts drop? Points good. With a running engine, you now have spark and fuel. It had compression. You did not break any thing, it would have made some expensive sound for sure. Now, you have fuel, spark and compression it should run. I hope this helps. Let us know what you find.
