If I remember correctly.
Take a look on the drivers side of the engine block under the heads. There should be a number letter combination first number is year, letter is plant, next three numbers should be engine size.
If the carb is bad before you spend a lot of money on it check out other options like a 4 barrel or if aftermarket fuel injection is available. I remember changing a 340 to a 4 barrel in the 70's. It helped performance but can't remember what it did to fuel mileage. The trick was to find a manifold that delivered low end torque.
Steve