Not sure what a "key off exhaust fire is unless your talking about turning the key off then back on to make it backfire and for me that was fun 20 years before this truck was made.
So I am in Cali and putting headers on that engine would take an act of congress to pass smog
I thought that I read something in the chev manual (posted in the tech section) about stainless steel manifolds, Ill have to see if I can find out more about them