Ive herd that Mac quility is preaty poor but ive never had any experience with them. I recommend going with Hooker headers. They have been making headers for a long time and the quility is top notch. Thats why I went with them.
There is another header called Kooks I believe and they are supposed to be top notch. I can personally vouch for Hooker from past experience as being very good.
i've had mac headers on 2 different gt's of mine without any problems at all. they have two different finishes now, the "ceramac" & hts coatings. the ceramic coating is obviously better, but they still have a decent price tag. macs also make really good power