Congrats on the gears....
I think that an Abbot ERA speed compensator will do the trick for the speedo problem.
Unfortunately, I don't know what the Saleen comes with... I'll go through the order I usually recommend:
Performance
1. Gears
2. After-market shifter
3. Pulleys
4. Cold-air induction
5. Subframe connectors
6. After market MAF
7. Sticky tires
8. Upper\Lower control arms
Looks:
1. Eibach lowering springs
The TB probably won't give you much, and the exhaust of the Saleen should be pretty open already, I wouldn't bother with the headers quite yet. (I've also heard that they can be a pain in the rear end)
Some or all of the above may be included in the saleen kit, so modify if needed.
The only other thing I might add that wouldn't cost an arm and a leg or involve engine work would be a small shot of nitrous.
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Hey, you weren't supposed to be that fast!"- Turbo Eclipse Driver