Beyond what the stock rotating assy can take? Forged internals/ported heads and new cams and then a power adder to take advantage of it and then a built driveline to get the power to the tires and then fully going through the chassis and suspension so the power at said tires actually goes to the pavement. $12-$15k plus whatever you paid for the car before factoring in labor if you aren't diy.
If you don't need more than 425 rwhp, Vortech, bolt ons and go.
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2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.