Unless you're pushing 300 hp, which you aren't with just a cam, TB and pullies, fuel starvation isn't your problem.
Fuel starvation wouldn't be an issue at low RPMs, either.
As far as running better with a different fuel pressure, it will only happen immediately after you bump it up or bump it down. After driving it for a while, the computer will recalculate its adjustment tables and compensate to go back to the computer controlled air/fuel ratios.
I'm assuming that you still have stock heads and a stock intake? A 70mm TB is way to big and the cam is a mismatch.
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351W 89 Mustang GT Convertible