I installed my header, along with the liquid heat riser and it works great. If you install a header and don't get even small power gains and you lose MPG, then it's because you didn't re-jet the carb after installing the header.
I also didn't have to go to any extremes to get my header to seal up without leaks. Just used a standard gasket and made sure I torqued it up evenly then hit it again when it was really hot.
It's a matter of personal choice, especially since you already have the header. Pull off the manifold and give the header a try. While you're doing that, get the manifold fixed, that way you can always swap back if you want.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang