I got lucky installing the Masss-Flo EFI on the big block in my ERA since they not only already have a return line bung (plugged) on their tanks standard but they also have a connection for an electric fuel pump included in their wiring harness. All I had to do was to plumb in the pump, make up a new return tube for the existing bung and run the lines from the regulator back to the return tube. All in all a very clean installation.
Used -6 all the way through (feed and return) and it has performed without a problem.
These are the feed and return lines from the return line side of the tank. You can just make out the inline pre-filter on the feed line on the far side.
DonC