In my limited experience, and it may be different for petrol, the fuel breathers are rarely large enough to deal with fuelling full bore from a proper fuel pump. My advice would be to see what is supplied/recommended with the underdeck tank, and then double it, and if there is room, double it again. There needs to be enough capacity for air to escape as fast as you put the fuel in.
Otherwise, you will be sitting on the fuel barge dribbling fuel in, with a huge queue of boats behind you!
PS. And if you can get it so that you have a dipstick, instead of/as well as, a fuel gauge even better!