Any changes you make to fuel pressure may yield you more horsepower on the dyno, but once you start driving your car that power will just go away because of the adaptive strategies built into the computer. Changes you make to timing will be permanent so I suggest you concentrate on that.
If, however, you decide to tweak fuel pressure, make sure that the dyno facility has a wideband O2 sensor so that you can find out what your optimal air/fuel ratio is. You can then get a chip burned to adjust your air/fuel ratio to what is optimal. Having that chip will then make HP changes from air/fuel adjustments permanent.
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351W 89 Mustang GT Convertible