Update : On 3 turbo kit.....After lot's of hard work, and using the Holley X tuner, my son finally got the T Bird Fox body car to start and run, somewhat?... It didn't run well..... He finds the ON 3 exhaust manifolds both leak in the upper sealing area....What a disappointment after many hours of labor !!!. He had to remove the exhaust manifolds and do lots of welding to ATTEMPT to add metal to the outer edges and still keep them flat....He can only hope for success?....He has spent countless hours on the installation....(Keep in mind that he had to do modifications in the 1st. place and contacted ON 3 about issues long ago with no results).
Personally, I have always been leery about header flange leaks and know some leak and some don't... Maybe stock or after market cast iron would seal best?...He feels cheated after all that hard work... NO MORE ON 3 for him!!!!