Only thing from the factory that came with 3.55s were the Machs and Terminators. That's it. Whoever is telling you that yours might have come with 3.55s likely has their head up their ***.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.