Craig - a 4bbl, headers will help but those old Fords had a head problem, the ports & passages weren't exactly smooth for flow. A cam will only open the valves more & longer (lowering your mieage) Try advancing the timing a few clicks that always helped. A high flow air cleaner, 1 step colder plugs helps, and synthetic gear lubes in both diffs & transfer case will increase the MPGs.
Re: Dyrtes" idea of acetone - it's 1oz per 5gals of fuel and that's not a guarantee but it does help on some.