Thanks for the posts, guys!
I kind of figured it wouldn't sound amazing without a header, but I'm sure it would still sound infinitely better than the current setup.
Magnaflow was the first to come to mind. I'm gonna listen to clips from those as well as another type that I can't remember at the moment.
And I believe it's original. Eveything on the car was original when I got it. If it isn't, it's still a really old muffler.
I was also looking at Classic Inline's site, and i'll definitely have to save up for the header and labor. It'll be further down the line, though.
Does anyone know any kits off the top of their head that don't require a muffler shop, though?
1966 Mustang, Automatic, Inline 6
Bought, not built - 9k, on August 22, 05
"She's my time machine, She's my rolling memory,
She's my family, And I love her so.
Every mile's another song." - (Five for Fighting - 65 Mustang)