I do not really understand why you want to go from return to returnless but if you end up doing so then I suggest switching to a Ford truck/SUV style of returnless which is not electronic. The Mustang's have a FRPT (fuel rail pressure transducer) that feeds the PCM data on the pressure drop across the injectors. The PCM then inputs a given voltage to the PWM fuel pump based on its fuel pump voltage table to get to target rail pressure. With the early 2000's trucks and SUV's they had returnless but it was mechanical. All pressure regulation was taken care of in the fuel tank itself before sending the fuel forward. This will end up being a much cheaper and easier option for you do to.
In fact, not meaning to make some sort of sales pitch, but I have damn near all of my stocker fuel system from my 03 Explorer. I swapped to a custom A-1000 based return fuel system for the Kenne Bell setup on it a while ago so my entire stock fuel system had to be eliminated.