can not make up my mind need your help

I believe it'd be in your best interest to get at least subframe connectors and possibly more bracing before adding any more power. I'm assuming your car is a hatchback because 5.0 and GT are in your name, and the hatch's especially need bracing because they flex like crazy. What do you want to use your car for? spirited handling, drag racing? After bracing i'd recommend getting the suspension you want, like Bowtie eater mentioned. You shoud be able to get springs and subframe connectors for under $571 dollars. If you decided to get the GT40p heads/cobra intake, you would also need to get new headers because of the GT40p's relocated spark plugs. That intake/head combo should give you loads of torque and great horsepower. Since the car is apart when replacing heads, you should also get a cam...which of course is more $$$. That's why i'm recommending before spending money on power, you should get chassis bracing and suspension which would more fit your budget and would mean your car can stick to the road better when you later get the power you want. Have fun and good luck...enjoy the money while it's still there