The Cobra should have come with a VSS. If it didn't, you will need to get a VSS. Any VSS from a 94-98 Mustang should work for you.
Then you will need to use a Dallas Mustang Speedcal or an FRPP Recalibrator to upscale the speed signal from VSS rates to the OSS rates that your PCM expects.
It will be helpful to know the tire revs per mile (tire size) and the axle ratio of your Mach. With that information, plus the VSS driven gear tooth count, you can calculate the Speedcal/Recalibrator setting you will need to use to scale the signal.
You can read a lot more background information on my web site here:
VSS/OSS Swaps
The calculators you will need to use are also on my site:
Ford VSS Calculator
Recalibrator & Speedcal Calculator
