the red does not hold up to heat
the permatex copper is high heat and has seal all my mannys with no gaskets
I use it on headers with gaskets also
just let it cure 24 hrs first
might want to check that the exhast pipes are not to ridged
as the engine torques over it can cause them to blow a gasket or crack a manny
I use a flat file to check the surface on manny and the head at the ports to make sure the are flat
never had a hemi...but I have had lots of junk exhast systems
the copper has fixed some pitted up header flanges more than once for me