They are designed for the 360/340 ports. I used felpro gaskets on mine and haven't had a problem. Can you feel where the leak is? Are you sure the emission ports were sealed up good? What did you do to seal them up?
Also most header companies recommend not using the headers for breakin's because that is usually the time you will find leaks and they don't warrantee them if the coating is ruined by an oil or antifreeze leak.
They say to break your engine in first (for new engines) before installing them to avoid any leaks or spills ruining the coating. I found out the hard way before they actually warned people.
Find a mexican muffler shop. They will do it for a 3rd of that. I would bring my own X or H pipe though. We have a Vasquez and sons here and they beat pretty much everyone on everything and they don't mess around.
I found this link & was wondering if it's possible to get them cheaper for my small block A body.
http://www.chicagoconnection.us/index.php?main_page=product_info&cPath=21_133_442&products_id=1922