You should be fine then.
Here's why it's simple, there's
Sensor wires (some are sensor supply wires: +5v, others return the signal to ECU, others are routed through the main harness to supply the gauges)
(at least this is applicable to our cars, newer stuff borderlines black magic with data lines and such)
You get the wiring diagrams/pinouts and run through the wires, matching them wire for wire.
I'm not familiar with the EVO8 ecu wiring. To get it running (after connecting everything to the engine) you should only need to apply +12v, Switched 12v, and ground to the ECU.
To get the gauges to work, you'll need to jump the sensor wires from the ECU harness to the chassis harness. I'm not sure if the EVO ecu is programmed to use the stock sensors, or if you need to add some. This is where it might get a bit tricky, as the gauges are designed to work with the stock sensor. You can get programmable gauges, get gauges with their own sensor, or simply keep the stock sensor & double up and add the extra one the ECU needs.
The start/charge harness is mostly independent of the engine harness. (which is really nice) There is a plug that jumps power to the engine harness (i think for the injectors) but the folks providing the new harness for you should have the pinout & power requirements.
The only extraneous thing I can think of that may not be accounted for is the radiator fan switch/relay, I think this is part of the engine harness, but I'm having a hard time remembering how it's routed. This isn't a big deal to sort, though - and the EVO ecu may have a programmable/digital trigger for this anyway.