Charles,
I have assembled several GMC inline sixes in recent years and had good results from using both Felpro gaskets and BEST gaskets.
I have been using cast iron headers (Nicson) and have not experienced any leaks in over 20,000 combined miles on two of the engines.
Thanks Ken for the reply,
I If I go to headers I may not have that problem, however I'm keeping the stock manifolds and there seems to be my leak issue.
Many Thanks,
Charles