Ask them for a Wells part number TU-5. That is the correct sender. The ones on eBay are pretty much all the same incorrectly calibrated "one size fits all" pieces of crap they sell to enthusiasts thinking we will never know the difference. But once you install them and then see your gauge reading 210 when it is actually 170 degrees water temperature, you will get as mad as the rest of us.
The Wells TU5 is the most accurate one I tested other than finding a vintage 1970s Delco sender. Stay away from the newer Delco senders as they are completely inaccurate. See here:
http://www.sssacc.org/Tips/Temperature_Sending_Unit.doc.
Or here (where they used my chart!).
http://www.camaros.net/forums/showthread.php?p=1486927
It seems like all the modern produced senders other than Wells are made by the same company who has the resistance settings all wrong, leading to the too-high temp gauge syndrome.