Your best bang for the buck would have to be a gear swap. If you have a 5-speed go for 3.73's. If you have an auto go for 4.10's. The gears will run you about $200 and installation will be another $200.
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1990 Mustang GT 5.0
1966 Mustang 200 six
2001 Ford F-150 5.4
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