[StoplightWarrior]

You need to get your speedo recalibrated, but you don't need a chip for that. You can just get a new speedo gear. Im almost positive on that one. I wouldnt go with wimpy 3.73's though. The mod cobra's love gear, so go with at least 4.10s if not 4.30s. As far as the install, it shouldnt be more than $300 at the most + the speedo gear. The X-pipe is a great mod, as are the k&N, shifter and pulleys. With all those mods intalled, you're snake will show stock vettes it's taillights.
Those mods are tried and true, and not too expensive:

Gears + install: ~$500
Pulleys: $200
Filter: $45
X-pipe w/cats: $450
Shifter: $200

So that's about $1400 in mods, and you could definitely pull a 13.1-13.2 with them in place. Stock vettes go about 13.3-13.7. Good luck.