Corral.net has some great deals. But if you can't find anything like that, look into some gt40 heads and intake. You'll want to do the springs and rockers since you've got an aftermarket cam, but you can usually find the parts cheap, and there is a noticeable gain of power.
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Or, look for a used vortech kit around that price range. You'll make streetable power all day and it'll be very reliable.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***