You can port the original heads and gain around 20-25 hp, or get the FMS heads (alone, they will get you around 30 to 35 hp)
I took a look at your mods in user's rides.
For the money that you would spend, I would be looking for a safe shot of nitrous.
(or a supercharger if you've got the cash...)
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Hey, you weren't supposed to be that fast!"- Turbo Eclipse Driver