Good thoughts both. THanks guys. I'll check the flange and Greg, you are probably right, since I can get head and thermostat gaskets and the like at NAPA, I bet they have metal ones for the Buick motor.
Jeff , years ago I was associated with the Slagle Racing TR8s . we ran high temp silicon sealer on our headers. These were individual tube headers that slip fit into the collectors not a complete length flange with all the tubes . Not sure if that will help you but it is a point of reference .