if the plugs were all soot covered I'd be concerned about to rich but you need to pull them after a 10 to 15 min drive on the highway....not like after the car has sat overnight then fire it up for 3 to 4 min...cause it'll be running in open loop still and is programmed to run a little rich until it reaches operating temp and switched to closed loop where inturn it'll run off of all input sensors to ecm......in open loop richness will vary based on temp outside..the colder the ect read and act read the richer it'll run the car in open loop for quick starts and smooth running in the warm up phase...but if you thunk you have a constant rich problem..you need to perform a koeo/koer test and check for any codes or issues the ecm may or may not be detecting and if none..from there I'D CHECK FUEL PRESSURE, and the fpr = regulator for a ruptured diaphram..evident of the vaccum line being saturated with fuel..and from there if you have a maf equiped mustang..clean the maf with the appropriate cleaner found at any parts store