All the items suggested above are worth checking. My long term observation has been that of those listed the one best able to provide what appears to be good performance and still slug the gas is an incorrect carb mix setting. I on several occasions have quickly doubled or more gas mileage just by tweaking the adjusting screws. As the '51K has a two barrel carb, there are two adjusting screws, and they need to be synched in order to obtain maximum performance. Procedure is to attach a vacuum gauge, and then fiddle with the screws until an optimal reading is obtained. Would think that any other problems affecting mileage this much would be hurting performance as well. Of course a fuel leak ahead of the carb would do this nicely too, along with burning your car up at some point. This might not show up with the engine not running, so need to check exhaustively while it is.
If you have put on a few miles since overhaul, pull a plug and see if carbon is forming. This will indicate too much fuel or too little air.