Well, it you're trying to run a two barrel throttle manifold bracket, the mounting point for the cable is gonna be too far back, which will make you cable too short. (Think about it; the front two barrels of a four barrel are located ahead of the two barrels of a two barrel.) Combine that with trying to attach the cable to the wrong point on the carburetor, could easily make your cable too short.
In other words, maybe it's not that your cable is too short, it's that your other pieces are wrong.
If you have a welder, a two barrel manifold throttle bracket can be modified to move the cable mounting point forward. Alternatively, you could spend money for a four barrel bracket. Or, as suggested in post 2, buy an aftermarket bracket that will let you move the cable mounting point around. And while you can get by without the throttle lever extension I showed you above, it's much easier with the extension. If nothing else, it will give you the proper attachment point and the correct amount of travel. (Think about it; the throttle lever travels in an arc, so how much the carb opens depends on where on the lever you hook the cable - lower more, higher less. So - mount the cable too high, and you'll need a longer cable. Mount if in the correct spot and a shorter one will do.)
