Pressure side (from the thermostat housing) should go to the bottom fitting on the core, suction hose to the top so the core remains full.
It does sound like a mixer issue though.
Get in touch with Kev from process west. I'm sure he'd like to see the tank that failed.
If the weld was sound and the tanks had no external impacts/damage there is no way fuel pressure would be able to cause it to fail.