There's a HUGE difference between an 03 and an 04 Mach 1
1. The 04 has 40th anniversary badges on the fenders
2. The 03 valve covers are painted black, on the 04 they're unpainted
3. Screaming Yellow replaced Zinc Yellow in 04
4. Competition Orange added as a color choice in 04
5. Some people will say the 04 is 310hp and the 03 is 305hp, but according to Ford, it's 305hp 320tq for both years.
6. Some say the exhaust ports on the 04 are different, but I haven't seen first hand evidence of that.
All in all, the 2003 and 2004 Mach 1's are basically identical