A thought on headers. This may not apply directly, but I remember this and thought I'd share. Years back, I recall seeing a dyno test on, I believe, a 289, where they were comparing exhaust system components. Obviously, the standard 289 2v / 4v factory manifolds were restrictive, and produced the lowest HP numbers. Then they replaced the stock manifolds with a set of Hi-Po cast iron manifolds, and the numbers were about the same, I was suprised a bit. Next, they bolted on late model "shorty" headers. The numbers were still shy of being worth all the hastle with the "shortys", but for space limitations I see why some people chose them. Finally, the dyno guys installed long tube, full lenght headers on the dyno mule. The horsepower now jumped to where you would actually feel it seat of the pants.
Certainly a Cleveland is a different breathing animal from a Windsor Ford, but, although your Cleveland may have the 4v big port heads, the exhaust side is weak in comparison. The heads were cast as a sort of comprimize on the exhaust, due to the front spring tower being right there next to the engine, and Ford had to cast the exhaust port angled down to clear. All race head castings re-shape the port, or raise the roof to rectify this shortcoming. So.....Yes, even if you have the "good" 4v heads on your Mustang, headers will improve the characteristics of the exhaust breathing. If you have a trailer queen, or a dough-nut shop warrior, or only use it to go to parking lot car shows, you may never use the benefits of headers. If you drive a lot, hit the freeway often, and of course, if you race your car, the benefits should be there for you to feel. If you aren't racing, try to get the smaller sized primary tube offered, to keep scavenge velocity up in low and midrange useage. If you do make changes to your Mustang, I'd always keep the original parts whenever possible. They don't make the manifolds for these ( or do they? ) anymore, and as the years go by, you may sell it, and the next owner may be very thankfull that you didn't get rid of parts they may want on the car.
If you don't want to modify your Mustang, and like'em original and factory correct, I can dig that too!