[Lightfoot]

I think you're just limited to the later model dual jets for that intake, but I could be wrong. Possibly an older rochester 2-jet might bolt on but I wouldn't know without physically trying it out myself.
Regardless, I think you should be able to diagnose the problem with your OEM carb with a little determination. Seems like it should be something simple, that was possibly botched up when the carb was rebuilt. I wonder if the needle might be stuck in the seat. Or (Same Thing) the float is stuck in the "UP" position.
Have you sprinkled some gas down the throat of the carb to see if the engine will start for a few seconds? I would start by filling the bowl through the vent tube with a tiny funnel. Then manually pull the throttle while you look down the carb to see if the accellerator pump squirts.

There are lots of knowledgable people on here. Keep giving us feedback and we will try to get you up and running.

BTW 1978-80 Grand Prix are nice cars. They just get a bad rap because it was at the hieght of the smog era and first downsized GP.
Hope you get the problem fixed and stay on here for a while. Post a picture of your car if you have time.