The speedo signal is created by a mechanical gear just like a cable speedo. The gear drives a electronic sending unit which converts the gear rotation to a signal for the ECM. This signal is the same as any electronic speedo signal so even if you have a fancy project car without a ford ecm you still have no worries, you'll just need an regular high quality electonic speedo.
the '98 mustang's speedo is not a typical electronic speedo at all... because it is driven by the ECM and not directly from the sending unit at all...
a mechanical cable can be installed right into a t-45 with no problems at all. Keep in mind that cobras and gt cars had trannies with a different number of teeth on the internal speedo drive gear so check your speedo accuracy if you are doing something like this.
etc etc etc.
I visit the forum rarely but feel free to email if you need very specific answers. I'm happy to help.
Adam Crabtree
adamc@bloodenterprises.com