serious carb issues

Sounds like the carb was modified off the car and needs to be tuned on your car by someone very familiar with them with some squirters, jets, power valves on hand to play with.
If it ran okay with old carb, you should be allright once you get this one dialed in right would be my best guess unless this carb is just plain too modified or worn out already.
Got any buddies good at tuning carbs with lots of tuning parts to play with?