Your tuned pipe needs a pressure nipple like this:

http://www3.towerhobbies.com/cgi-bin...?&I=LXNFB2&P=7

You need to install it just aft of where the pipe reaches it's maximum diameter - just aft of the bend where it goes from an expanding tube to a constant diameter tube.

Best if you can find some way to build up the area that you drill & tap into so there's more than just the skin of the pipe holding it in.

If you are using a silicone coupler like the one in the picture you will probably have to replace it often. Be sure to clamp it down well or the pipe & header will slide apart.