Not sure about Stewart Warner electric guages, but with Autometer electric guages you can get various fuel guages. For the Mustang, the stock fuel sender will work. You just need to insure the guage reads 73 ohms as full, 10 ohms empty. You should not need a special sender.
The speedometer requires a seperate sender that is not usually included and usually run about $60-$75. Autometer will work with the Ford VSS used in the 86 - 93 Mustangs ( ~$16 at Autozone). The Ford VSS has dual outputs, 2 wires to feed the computer which can also feed the electric speedometer, the other output is for a typical speedo cable.
I just left the cable off, but put a small plug in there just to insure fluid wouldn't leak from there.