01 might have 19lb injectors vs 21s for the 04. Also if you got a REALLY early 01 it might be a Windsor. 13 valve cover bolts=Windsor, 11=Romeo. Only other difference is 04 heads had 8 thread spark plugs holes vs 4 in the 01.
Performance wise, identical for all 99-04 2V GT engines.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.