Car Classification Rules Seems to confirm the Street Mod classing. Your LSD is allowed in STX, but your 3 Liter is not. It is specifically allowed in SM, however, as long as it is from the same manufacturer (Ford to Ford, but not Jag to Ford, or Volvo to Ford. Chevy to Ford is right out.)
I was also thinking about Street Prepared, but that would work only if the allowed overbore (.0472 in. over standard) put you at the actual displacement of the 3.0. And then, you might be pushing it.
If you just want to have fun, I'd say SM for sure. At least around here, it's a more popular and open class, and more entrants is always better in my book.
