The plastic tanks FIT just fine,and have the return line. Problem is the gauge sender. They are different Ohmage and are reversed electrically,IE if 1= Empty and 9=full, the plastic tank sender is more like 9=empty and 1=full. I have found a way to adapt the old sender to the plastic tanks sender,and can take a pic if you need one.
As to the wiring of a turbo engine...use as much of the Merkurs harness as possible. As to connecting it to your truck,I've always used this diagram as a guide,for everything from adapting a harness to building one from scratch:
http://www.blindchickenracing.com/How_to/T-bird_install/t-birdwiring.jpg
Good luck to you. I don't visit here daily,but would be happy to help whenever I do.