While returnless is a more precise way of fueling, as is evidenced by Detroit's adoption for emissions purposes, it is definitely more complex. You will find your project will run into different obstacles, perhaps with increasing frequency, as the availability of period-correct aftermarket replacement components becomes more scarce for the factory ECU you are using. At some point, you will find it attractive to replace the factory ECU with an aftermarket unit that is more capable, flexible, and tuneable.
The one that I regularly suggest is the MS3Pro Plug and Play unit that allows the use of OEM dashes and wiring harnesses. You have already invested in a Ron Francis wiring solution. The PnP version of the MS3Pro ECU would still plug into the OEM connector on the Ron Francis harness, so you would not need to rewire the car yet one more time. Don't forget the OEM ECU was not designed for a performance application. It was designed to meet demanding (at the time) emissions standards. Other goals were secondary at best.
