No SS 396 ever came with a 2-bbl carb. Either the guy has no idea what he is taking about, bs'ing you and he swapped the manifold to save gas, has another engine in the car or the car is really not an SS 396.
Best way to tell what you have is to get the engine codes stamped into the boss of the block that protrudes from the right pass head. You will have to remove alternator and clean aways paint and grime. When you do this, look closely before you do to see if this area has been painted up before or if it appears original. If it has been painted up, that a good clue someone had been fooling with things.