Seeking a S10 Fuel Delivery GURU

I have a 2003 S-10 that came with a 2.2L and configured for flex fuel.  I am replacing this engine with a 2000 S-10 4.3L V6 with engine wire harness and ECM.  I am being told that the plastic tank, lines and sending unit must be replaced.  I see that the return line must be extended to the intake. What is the issue with the sending unit and my wire harness?  Apparently sending unit will not communicate and a proper sending unit for this harness and ECM is not available for plastic tank? HELP!!!