Sometimes a new gasket will shrink after it gets hot.
My experience on a downdraft:

Installed new gaskets
Torqued to specs
Ran engine till hot
Exhaust leak...expletive!
Let it cool down a little (too hot for fingers)
Rechecked torque..."loosey goosey"
Retorqued
No leaks

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."