If you were running lean with a stock fuel pump, why did you get a new fuel pressure regulator instead of a new fuel pump?
You also can't adjust the air/fuel ratio with an adjustable fuel pressure regulator. Your computer will simply shorten injector pulses to compensate for the increased volume of fuel. Your mods are not nearly extreme enough to require so much fuel especially since Cobras have 24# injectors.
How have you tested whether you're lean or rich other than by smelling your exhaust? Since you have a fuel pressure gauge, what does it do when you are at WOT? Does it rise to 49 psi?
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351W 89 Mustang GT Convertible