I would try a dab of the RTV, and do not tighten fully. In fact, lay some .005-.010 fishing line to space it correctly. Believe it or not, that was the standard operating procedure at TRW to control bond line thickness in advanced composite space vehicle structures. Let it set overnight, then tighten with the Nordlocks. What some enterprising person should do is CNC a shallow groove around the port on the header and insert a controlled crush copper wire in the groove.