I've had a custom A/C system in my GT for about 6 years now, still blowing cold here in Florida!!!!
If you want to stay totally stock I wont be much help.
Don't know of any new after market system for a GT. So were stuck with the ARA or GM unit.
The ARA A/C consol and rear blower unit is the best design ( got mine from Opel-gt source, thanks Gil ) and a must for an easier install. Yes there hard to find in nice shape and costly. A little bondo and paint on a cracked one will work if your careful. Hey if you live in a hot climate its a must.
Getting help from a local Vintage air is a good way to get ideas for a condensor and fan design and having custom hoses made.
Mine has a nose mounted condensor unit with a pusher fan and relay, this way the radiator still gets good airflow and the car looks stock!.
Now the hard part:
I mounted a Geo metro compressor on the (RIGHT) side of the motor! Yep, can't even see it!
Designed and built a custom bracket that mounts to the 3 existing holes on the block, a tensioner bracket, double crank pully, fan spacer, modified alternator bracket.
Tight, but, yes it will fit!
The small compressor drops RPM only about 100.
Need 50amp Alternator minimum.
Car runs cool.
The best part it looks stock and parts are available.
Lyle