360 running on 4 cylinders

First thing to do is see if/ which idle mixture screw is effective. Then try removing the opposite one and put your finger over the hold with it running. If that doesn't change anything, squirt som carb cleaner and then an air hose into the hole. It might not "fix" it but it might change things enough to insure that you are on the right "trail."

Back in the days when I had a SB Mopar in an old beater FJ-40 Landcruiser, the tank had a lot of rust issues, very fine powdery rust. Hell I ran three fuel filters on that thing sometimes, the powder would go right through them. There were "a few" times out in the woods when I had the top off the carb trying to get it clean. Used to run an AC sediment bowl filter at the carb. You cannot even buy those I don't believe, anymore.