Engine Probs?

Ok i have been trying to get this engine running better. It is a rebuilt 360 with electronic ignition system. I have a holley 4 barrel carb. The engine is running really rough. The car had been sitting a few years. When i put the motor in the car i siphoned the old gas out of the tank and then filled up the tank with fresh gas. This whole time ive thought it might be the gas tank so the other day i used a small gas can to run the car on. The engine is running the same off fresh gas in a 5 gallon can so that is not the problem. I checked the spark plugs and they were fouled. I cleaned them up and it still runs the same. The ECU box is old so i was wondering about it. I do have some exhaust leaks around the manifolds. What are the most likely things i should check and how do i check them thanks for the help