I did a Hayden 403 and the kit included 3/8" rubber line. I just went into NAPA and got the copper connectors - they were labeled as air hose connectors.
I know we've gone back and forth about which line is supply and return. You must have checked yours somehow? Maybe gm switched up the direction on us.
This website seems consistent with my car. http://www.grandprix.net/upgrades/cooler.html . I just don't want you to install the wrong way!
I used the standard worm/screw clamps and they have been great - not a drop leaking. I read to avoid over tightening them; stop once the hose bulges out flush with the metal band on the worm clamp. I think people tend to over tighten them and it cuts the hose.
Glad I could pass along some info
