Just in case it helps anyone doing the same kind of stuff, this is what I did.
I put the 3/8 nylon EFI line for the feed and return both and a pressure gauge in the feed line to watch the pressure.
Then I added a restrictor orifice in the return line until the feed pressure was where it needed to be. (5-6lbs) in my case.
Then I took the gauge back off and put it away, and if the pressure were to vary to the point it caused a problem I would see it on the A/F gauge.
Been that way for close to a year without a single problem with it.
But to answer the question on using the old feed as a return, yes you can and it works just fine.
