[strictlypersonl]

The FIA tank sits above the rear suspension, the 427 tank is behind it. They share no parts and are a completely different design.
The tap on the FIA tank is on the center/bottom. It's difficult to put a return line in any place but proximately close to that one. The other alternative is to install a separate tank somewhere that takes a feed from the main tank and runs two lines to the front: feed and return. There's some room behind the left rear wheel that might be OK for a custom tank.