This set up seems a bit strange to cause such a big problem. We're only talking the extra thickness required in a washer of half a turn of the thread pitch, which is going to be well within the size thickness of a couple of fibre washers, unless the tank and taps have some outlandish coarse thread. In which case a soft metal spacer, aluminium, copper, or lead plus fibre sealing washers top and bottom may be the answer. You could take a dose of courage and skim half a turn of thread off the face of the tank tap spigots.....but only if its a pattern tank. Original tank...leave alone, try another tap, or back to plan A.
Anyone know if any of the Loctite products are Petrol proof? If so, just mount and leave to set. Might work as a quick temporary fix, but not robust enough for hard use alone.
Swarfy.