After you get the vac advance working. If you happen to keep the rochester, carefully take off the top of the carb. If the fuel level is near the top, adjust the float so it shuts off sooner about 1/8 inch lower. Too high will give you sooty plugs. Check the new jet you put in from the good kit, maybe too big. Put the old one back in. Get the engine hot and shut off and set the solid lifters~valves to specks. Valves set too tight will give you poor gas mileage such as you are experiencing. Friend of mine has a 50 1/2 ton GMC 228 like yours. We put in a Nova 3:08 rear and a 59 chevy 3 speed, bolted right up and he is now getting 18 mpg. Perfect gear for Calif and the desert.
