X2- what Doright & Bruzilla said. I've got a 74 Charger 318 2bbl, all stock except for dual exhausts. I've gotten some grief from people who say, "Oh man, you gotta put a 440 in it, your car is so slow..." Sure, my Charger isn't going to set any speed records. But it gets above 15mpg on the highway, runs smooth, never lets me down. If you're going cross country, the more you leave your car alone the happier you will be in the long run. Headers and a dual exhaust on a 400 is enough. Trust the people on this board, they have been around the block a hell of a lot longer than you (or me) with Mopars. IMHO, the best thing about driving an old Mopar is that wonderful contented feeling you get when your car is rumbling along the open road, and all is good with your world. If you mess with your car too much, you will not experience this feeling because you'll be too busy trying to make the car work with all the crazed modifications.
If you really need to go fast, many motorcycles under $5000 will blow almost anything on the road away.