Yep, check the grounds!
Also, you can check the power to the coil to make sure it's not on the power side. These use the computer to 'trigger' the spark by controlling the ground side. Pin 12 of the smaller PCM connector in this 88 full size wiring diagram. Hopefully, you have the factory info for the Shelby.

http://ww2.justanswer.com/uploads/ASEMas...ing_diagram.gif