1988 Dodge Dakota Poor gas mileage and rough idle.

I recently purchased and installed a rebuilt engine.I installed new plugs, wires, cap, rotor, fuel filter, air filter, pcv valve, egr valve, and multiple vacuum lines.I have a slight hesitation on take off and a rough idle.I used to get about 300 miles out of a tank of gas, now I am getting about 200 miles out of a tank.I replaced the engine because of a blown head gasket that over heated and lost oil pressure. People have given me multiple solutions but none have worked. My timing is set at 10 degrees btdc and I am almost possitive all vacuum lines are installed correctly. My catilitic converter was new before I blew the head gasket and I dont know if it was damaged by the steam that was coming out of the exhaust when I blew the head gasket because it seems to have good exhaust flow.