Hd or Lowes type store will have the elbow which is called a 90 degree street el, get galvinized, then get a gas valve that is .375 as I believe that manifold ported that size.The newly installed valve will then be regulated partially open with the other 2 valves staying in the original run positions. I believe you will only have to open the new valve 10% or so but just play with it until the boil is a "lot" less. While your doing this consider those schedule 80 extensions.If you can get that liquid filled gauge you could put a T in place of the 90 elbow and run the relief valve on the bottom of it and the guage on the top.
