Your fuel economy will not be significantly impeded by the addition of 3.73's. Your top speed will actually increase with those gears. Without gears, you don't stand a chance at waking that 5.0 up, not with the AOD anyway.
You might see 15hp from headers and exhaust, but an off road or hiflow H will give you 15-20 alone. Screw the headers, you need the low end. Go for an off road or hi flow H pipe catback, and gears. A good high stall converter will cut your fuel economy a little, but the performance gains are well worth it.
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