Been There. Done That.
You have the right idea about how to handle your quandry. Simply hook up the in and out lines on your Carter (in fact that's the same pump I chose) and then block off the return line from the tank. Apparently, the third line had a few intended purposes. First, it was meant to keep the fuel and the pump cooler in the long run, by looping it between the pump and the tank. Second, it was intended to pressurize the tank to keep it in compliance with EPA rules. There was a third rationale for use that was to effectively act as a pressure relief / stabilizer for the system. That's what I read in the manual, at least.
All that said, I've been running my Carter pump in the manner described for 5 years now, with no adverse effects noticed. Go for it!