Chris,
First the easy question, The roof racks are magnetic and I picked them up in Europe where I personally tested them on many seven hour endurance runs at 190 KPM with no problems. I lived in southern Italy and drove 700 miles on the Autostrada, to and from the Alps on the weekends. You just have to make sure the roof is clean before you stick them down.
The Tail light covers are a lot more work. They are not really covers, instead they are complete sealed units that are designed for trucks and trailers, that use either flange or grommet mounts. They are DOT approved and come in either red or amber. I used red for tail and stop, and the amber for turn, just like the original Opel, only I did the common swap of putting the brake/tail lights on the outside so drivers behind me would not mistakenly think I was further away than I was. I also put a third brake light set up in the rear vents, and added the Destec LED's to my Mirrors.
The lights are available at
www.buytruckstuff.com or most likely at your local truck stop. For the front they were a very simple replacement. I just gutted the housing and then took one of the grommets that come with these light and trimmed much of it away with an exacto until it was small enough to simply wedge in to the housing. You could ad some RTV if you were afraid of it shifting, however mine are pretty tight and have not moved in the six months I have had them, and the car is a daily driver.
For the rear it was a lot more work. I ordered the stainless steel flange trim covers from the same place, and then drilled three more holes in them so they would look simetrical. The original Opel assemblies were removed and I noticed that they were held in by three screws spaced pretty close to what the truck light flanges had. I used stainless hex socket head bolts to attach to the car and three dummy screws to fill the dummy holes. I then had to fabricate a wedge spacer that would do two things.
First they had to plumb the lights perpindicular to the road because the rear of the GT tilts quite radicaly toward the ground.
Second they had to flare slightly from the size of the flange to the car body. This is because the outside diameter of the flange is almost the same as the inside diameter of the old taillight opening.
To make the flare I used a rubber plumbing reducer that I got at Home Depot that I think is used to join a piece of five inch sewer pipe to a six inch pipe. I then carefully traced the angle I needed and cut the flanges by hand.
Then the stainless trim rims had the words "TRUCK LITE" stamped into them so I took a 60 grit disk on my 4 inch grinder and ground the words off and continued the swirl around the entire ring, giving a brushed SS effect.
Unless you are really looking for something different and are willing to do all the modifications I would instead strongly recomend looking into the LED units that Destec has put together. I have his LED units in both my rear vents and my mirrors and they are top notch and well worth the money. His tail light units pop right inside your existing assemblies.