I have heard bad stories about MACs quality from some reliable sources. It appears to be true. The only think MAC i have is my off road h-pipe, and it doesnt fit real good, but other than that, the quality seems acceptable.
As far as your header crack goes, simply get the crack welded, probably cost you $5.00, or whatever kind of money they use in Italy.
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1993 GT/AOD
'93 Mustang GT