A popular upgrade for nPI equipped Mustangs is the PI swap. This includes swapping PI heads and the PI intake on an nPI 96 – 98 Mustang resulting in an approximate gain of 45 RWHP, but with an increase in the compression ratio to approximately 10.5-1 making these PI head swapped engines supercharger unfriendly.
Another common swap that involves less labor at significantly less cost is the PI cam and intake swap. The swap provides a gain of approximately 30 - 35 RWHP while maintaining the stock compression ratio. Because used PI cams and used PI intakes can usually be purchased for approximately $100 each, this mod is proving to be one of the best dollar for horsepower mods available for the nPI equipped Mustang.
After the PI, you should get some 4.10s. For the 4.6L Engines the 4.10 gears put the engine in the right powerband and the right time. I have herd 4.30s are amazing, and personaly, i would have gotten them if i would have felt 4.10s first. I love the 4.10s, i have really not lost any gas miliage and if you want to spray they still give you enough band to do it (although if you know you are gona spray or blow, go 3.73).
Good Luck with you new car, and congrats (if you buy it).