Space can be an issue using Dowty seals here - see photo from my A10 - there is just and only just enough thread to screw the return connection to the tank.
Here I had to use a fibre washer inboard - it is 18 mm bore - and a Dowty the other side - 1/4 BSP. Anything thicker and the pipe union would not have been fully tightened. Tanks will vary slightly so yours may have a bit more space.
They are great seals but usually thicker than the average fibre washer but worth a try as the Dowty seals are quite reasonable prices from a number of ebay sellers.