The ignition is pretty simple. All you need is a 70's Duraspark setup, distributor, coil, ignition box, and small harness connecting them. (You can use the newer style coil, but this way is very straightforward to get started) Then connect the red wire from the Duraspark box to a wire that is "hot" when the key is on. There are some good diagrams out there for converting older Mustangs to Duraspark. You can apply same info to your conversion (I did). The gauges and alternator wiring are separate fron the EFI stuff. SWMBO's 86 "5.0" still has the 2.3 computer under the dash, not connected to anything in particular I guess. It doesn't effect the function of the car.
One problem you have that I didn't is that you have a fuel pump in the tank. I believe you may be able to use an 86 or earlier non-EFI sending unit instead. Or you may be able to just remove the pump and modify the unit itself to make it a simple pickup. If you wish to retain to "dual" fuel lines, you can use an 85 GT fuel pump. It has a fitting for pressure and one for return.
There are lots more details and issues to be covered and, yes, you will likely be better served by asking questions on other boards that are more oriented to "late-model" Mustangs. (An 1983 GT is 20 years old, is that really "late-model"? :

OK,
Fox -body Mustangs.