Pattern tanks can be terrible in this respect, the oe fuel tanks supplied on the cars had excellent swirl pots that were filled by the return line, before excess fuel over and above what the swirl pot held simply got pushed upwards and over the top into the main tank, these swirl pots were made by Mercedes!!, but worked ace, plastic in design, exceptionally unlikely to be a in tank prob with these as the pipes are all solid, right from tank outside to tank inside into the swirl pot.
tabetha