Yeah, 3/8" flanges never gave me headaches.
Torqueing the nuts over and over until they stay torqued is what I recommend. The expansion and contraction from heat cycles "chews" the gasket and retorqueing takes up the slack developed from that. One bad issue though is from bad threads in the head. If the studs have been loose the aluminum gets beaten from the engines vibration and the studs pull easily. Healthy threaded holes can hold the torque but once they get a loose stud the metal fatigues and won't hold much pressure. Same thing goes with collector and downpipe flanges, make several torque passes.
Quick general note on headers.
For low end power/torque, short runners
For upper RPM power, long runners
And tri-y for anything in between
IF your budget is big enough and you want a specific RPM peak then take your time and tune for whatever that may be.
