So I began with the process of swapping in a MY05 dash into my MY00 GC8.
How do I go on about the wiring process?
Can I use the existing GC8 harness?
Can I keep the existing GC8 heater core?
The 05 AC's controls and plugs are different from the 2000's GC8, is there anything I can do to make it work?

I read ECS's write-up on the dash swap and he says that I should cut off the MY05 ac plugs and merge them with the GC8's harness.
And there's the problem, the wires that go to the plugs have less wires than the 05!

Also, what do I do about the gauge cluster? which speed sensor do I need? WRX's? STI's? or the one I got the harness from, the MY05 autobox 1.6L FWD?

Any info would be great,

Holy hell, no one has replied?

Alrighty, the dealerships where I am from are pretty cool about getting me some schematics and wiring diagrams.

You will most likely have to cut off the old connectors and re-wire them with the new harness. One connector will have more wires on it than the other. That's when you either find a friend with AllData(or something like that) or call your dealership and see if they will help you out. I know some dealerships "can't give out that information unless its another Subaru tech".

You'll probably be able to get away with using the same heater core (I did w/my wrx swap into my 2.5)

As far as your speed sensore nonsense, I couldn't tell you. Sorry

There is a thread about this swap into a 02. I just did this swap also. You need the heater core and blower motor out of an 05+ and all the plugs for the servos that are mounted to the heater core and blower motor and you will need a sun load sensor, ambient temp sensor and i just tapped into the ecu for the speed sensor and i am also using a sti cluster that has the temp readout. the 05 has more wires than the GC because you dont have the sun sensor,ambient sensor and the 3 different servos. It isnt that bad of a job but its just time consuming and you have to get the right diagrams.