The Mustang 302ci and 5.0L engine are one and the same. Tell us the part no. of the cam so we can help you more. If the cam is specific for that motor, the firing order will be the same as for a stock cam.
Ford changed the firing order of the 302 in 1985 when they installed the "Marine" cam in the first HO's. Hotrodders had been doing that for a long time. The Marine cam was actually a 351W cam. By using the 351W firing order in the old 289/302 motors you could run get an economical upgrade in power. The Chevy guys are doing a similar swap now to smooth out and improve the power pulses in the small blocks. The cam can definately change the firing order but it also has to be correct for the crank it is used with.
95 GT Coupe, FRP B spring kit, Bullitt Wheels, Hurst short throw shifter, SVO Cobra Intake, X303 Aluminum Heads, Cobra Roller Rockers 1.7, Equal Length Ceramic Headers, BBK H Pipe, FRP Low Restriction Exhaust, Bullitt Calipers and 13" rotors. American Racing CAI.