Well don't look to a straight 6 if you want better gas milage. You gotta gear em way up because they don't make enough power so you'll be screaming down the highway at 4000 rpm which I don't think will help your gas milage at all. Also, straight 6's like to put their pisons through the oil pan when you run them that high because they have a huge bore and stroke. Face it, you drive a truck, they get bad gas milage. Your milage will be lower because your engine is worn out, so you should just rebuild it with some more compression. Get the block zero decked and that should rais you half a point. Then mill the heads some to get the rest of that point. Don't go crazy with your camshaft selection and give the motor some more ignition advance than the manual advises. Most smog engines run 4-8 degrees to reduce NOx emissions, but they get the best fuel economy and power from 8-14 degrees advance.