I believe russell fittings makes a fitting that will allow you to push it on the stock gm fuel rails and be left with a female or male -6 end.
Since the stock v6 fuel lines probably have 5/16" and 3/8" pipe or straight thread on them, you might just want to cut them off and use some -6an to hardline compression fittings.
Then attach the rail fittings to the new ends on the feed/return lines.




ding ding ding, everyone to their corners