Your best bet is to get a factory service manual with wiring diagrams and fix what was butchered. It's all pretty straightforward except this:
- The gauges, except the speedometer, require +5v that is delivered by a regulator in the dash that steps the +12v down to +5v.
- I'd consider bypassing the ammeter gauge while you are in there. It can be a fire hazard.
- The fuel gauge requires the tank be properly grounded to get a reading.
- Don't overlook the bulkhead connections, where corrosion can play havoc with electronics.