By your screen name, is it fair to say that the truck is 4 wheel drive? If so, you need torque to get the big tires rolling. The 318 isn't the first choice for that but the headers do help as would a full 2 1/4" exhaust system. The cam is borderline for torque in a 318 but it will work. Its certainly a step up. Stock 318 cams had .373 lift! Really.
Engine Quest sells NEW LA and Magnum heads. If you are bucks down, a good rebuild and port job on a set of 1985-91 #302 heads is one option. they are small valve, small port but with bigger valves and porting, they will do.