That crack is in just the right place to put in a divider baffle to make a split manifold for dual exhausts. Otherwise, look for a replacement. There are offshore "clones" of the old cast iron dual exhaust headers, but they're pretty poor quality and don't fit well. The supply of original equipment single exhaust replacement manifolds and the Corvette six cylinder dual exhaust system dried up years ago. Good condition used ones are available- - - -try a "parts wanted" post here, and search Ebay also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!