Howdy back Bill:
Welcome aboard the best Ford Six Forum anywhere.
I thought I'd add my two cents to your question, too. It will be abit of a rehash of what Joe has already given you.
The truth is, there is no magic bullet to performance. Likely the best bang for the bucks is a good tune up with some additional ignition advance thrown in. After that, a well thought out comprehensive plan is the best value. If you know what your ultimate goal is then you can start adding bits and pieces toward that ultimate goal. Lay a solid foundation.
So, next, it would be helpful for us giving advice to know more about your vehicle and it's condition; to know more about your end goal, to know more about how serious you want to get. What is the casting code on the head?
Then, know that a header will be relatively expensive an show a small gain in an otherwise stock engine. Yes, they are very cool and have alot of eye appeal, but without the flow in the intake, head, cam and exhaust little is gained. A header is most worthwhile as the rpm range goes up. On the other hand upgrading your stock exhaust system to a 2.25" or 2.5" ID system with a quality turbo type muffler, will be noticeable throught the stock rpm range and improve MPG and performance, and be a step already in place when it becomes a good time to step up to a header.
Another upgrade would be to step up to a DuraSpark II ignition system from a '76 or later 200/250, or to spring for a DUI/HEI system from Ford Six Performance. Open the plug gaps to take advantage of the hotter spark. Either will be noticeable in starting, idle, cruise and performance over your stock ignition and again, be foundational for what comes next.
The "Biggest bang for the bucks" time to change the cam is when you do a short block rebuild.
If you want to start with a head off enhancement, plan a valve job that adds 1.75" intakes to your stock head, milling to gain some more CR, do a three angle valve seat and back cut the intakes. If the bottom end is solid and you don't want to go into it to add a cam, consider 1.6:1 rocker arms to gain a little more lift.
That's my two cents, for what it's worth.
Adios, David