If the current carb is the problem, perhaps. And I DO NOT mean that to sound like a smart ass answer. A properly tuned Rochester Quadrajet can deliver good mileage. The small primaries for cruising make it an efficient carb under light throttle / load.
But there are LOTS of variables for fuel mileage. State of tune (plugs, wires, PCV, no vac. leaks), weight of vehicle, aero (extra wind dragging spoilers or raised roof?), driving style (do yo udrive conservatively, or a little more aggressively?), driving conditions (cold weather with lots of choke operation, lots of hills?), gear ratio, OD trans? Is the choke opening as fast as it should (or is the heated crossover that powers the choke clogged...or is it electric and inop)? Timing (set to spec, or retarded to allow low grade fuel, further retarded to avoid spark knock from carboned up combustion chambers from leaking stem seals, or inop. EGR)? Vacuum advance working? Lots of things go into it.
But a worn out carb, with floats that are 1/2 sunk leaving it running rich? Sure, that will hurt MPG, and a rebuild would fix that.