On an 1983 Mustang, there are two temp sensors like the EFI cars. One is the ECU sensor, and the other is a temp sender for the guage. For your holly EFI, just pull the OEM sensor and install the holly sensor in that port assuming it's also 3/8" NPT.
Leave the temp sender alone and your guage will work. (assuming you aren't gutting the OEM wiring)