For your surge tank (nice fab) for the nuts you are having problems sealing, you could probably use aircraft 'Dome Nuts.' They are used in aircraft fuel tanks (in a 'wet wing' tank where the skin of the aircraft is the tank) for the access panels in the bottom (and the top) of the tank. They rivet in (to be fuel tight they will have to be solid rivets)so they are captured.
If that won't work see if you can get some aircraft fuel tank sealer. It gets called PRC or Flamemaster sometimes (brand names). It's not cheap, but it is fuel proof.
If you want a brace, could you put a tab on the tank that would line up with the screw on rocker cover? Replace the screw with a stud (I would prototype with all thread myself) secure the rocker cover with a nut, then put the surge tank in place and secure it with another nut.
