Ok, the freeze plug worked.
I got one that was just a HAIR too small to fit over the opening, then persuaded it in place with my favorite tool, a 3lb hammer.
I used a regular automotive muffler clamp (like yours, T-Mack) and took it out for a spin.
WOW, what a difference. it sounds SOOO much better. Can't really say that the butt dyno detected any more get up and go, but I didn't run it on the expressway yet.
I'm planning on taking it to Mr. Muffler tomorrow and having them weld the pipe to the header and welding the freeze plug to the cross-over hose fitting.
Heres a pic of the pipe installed. No T-Mack, It fits with no clearance issues...