Now I understand why you said you have multiple return lines, the one you want to put the flo meter in is the one that goes back to the tank not the ins that go tot the banjo connection on the fuel filter which if my memory serves me correctly would have been a CAV 7111-296 or a pair of them when those engines were in automotive applications.
Actually all three return to the tank so you have to combine them and then go through the flow meter. Probably an installation issue when the boat was built. Emailed Bob Smith at American Diesel, he was involved in the Lehman conversion, and I think he responded that the return from the head was extremely small and could probably be ignored. But I decided to combine all three since I was in there anyway. The biggest issue I have with the Floscan is the pulsation dampers ahead of each flow meter. The flow meters themselves are very small but the pulsation dampers are about the size of an automotive oil filter. Had not anticipated that when I ordered them.