While the Opel alternator bracket can be made to work, it works best with an offset bushing in place of the rubber one. I have in the past used aluminum, steel, teflon, poly, and JB-weld to fabricate the offset bushing, and surprisingly all worked about the same. (JB-weld needed an inner bushing from the hardware store.) The bushing should be made to accept the slightly larger GM alternator bolt.
The last 2 installs I have done have been slightly different. I have used the $12 chrome alternator mounting bracket from Autodrone, or any other local parts place, instead of the Opel unit. This is the one shaped like an "F" in cross-section and mounting with 2 bolt holes. The holes aren't exactly the right distance apart, but 5 minutes with a rat-tail file to enlongate the holes towards each other solves that problem. The front hole will get close to the rear mounting tab, and require using an allen headed bolt, but all the bolts on my engines have already been swapped to allen anyway, so no biggie. This will firmly mount the lower "main" tab of the alternator.
The upper tab of the alternator is slightly more difficult. Opel alternators have their mounting tabs set at @ 150 degrees apart, and GM alternators are straight across from each other. The Opel adjuster simply isn't long enough to reach the GM alternator's adjustment tab and will have to be replaced. The difficulty with that is the arc for the adjuster needs to be pretty severe to clear the alternator's fan. Chrysler vehicles with the slant-6 have an adjuster that works fine, and is easy to find in almost any junk yard. Get one of these, mount your alternator and call it a day. You may find that you need a slightly longer belt, too.
Alternately, and how I do it now, you can get the universal chrome adjuster that was hanging next to the alternator mounting braket and adjust it to work. It is @5" too long, so a new hole will have to be drilled farther from the non sliding end than the origional and the excess cut off. Now the braket will fit, but the arc won't clear the fan properly. Now is the only difficult step: Remove the fan and pulley from the alternator and replace the fan with one from a late-model smaller GM alternator. (The fan I use is from the alternator fitted in the '89 Cavalier and the like.) Lots of autoparts stores have extra fans and pulleys laying around that they will just give you if you ask, or you can take a 5/16 allen wrench and a 5/8 deep offset wrench with you to the junk yard and get it yourself. When I get alternators for Opels I just do the swap myself in the yard so I only have to buy the one alternator.
I would love to get some pictures for everyone, but the cord for my digital camera is MIA....
Now, the wiring:
The easiest wire to hook-up is the main wire for power output from the alternator to the rest of the car. This is the same red wire with the same lug as before but now on the new output post of the new alternator. The GM alternator has a built-in regulator, and the Opel doesn't, so the extra Opel one has to go. Remove it and the 3 wire harness from it to the old alternator, carefully cutting the one remaining wire to the regulator from the cars harness, leaving as much as possible in the car. Typically this is a light blue wire with a white stripe, and it is from the "idiot light" in the dash. Your new alternator has 2 spade terminals in the side near the back. I have no idea what colors you will end-up with for the mating plug for these terminals, but if you get the $4 new replacement plug at the auto store they will be red and white. Looking at the alternator from the back, the terminal on the left should be labeled (1) and the wire *should* be white. The right terminal should be labeled (2) and *should* be a red wire.
-Terminal 1 (the left one, should be a white wire) is for the idiot light and "turns-on" the voltage regulator, and that wire is to be connected to the blue/white wire from the GT.
-Terminal 2 (the right one, should be red) is where the alternator field gets it's power. This wire needs to be connected to somewhere where there is always going to be power, like the output terminal of the alternator. (When the alternator isn't spinning there is power on this terminal from the battery and when it is spinning there is power there from the alternator.) Put a nice big terminal lug on there and attach it to the output post with the other big red wire.
Now, assuming you mounted the alternator securely and wired it as above, the car *should* operate exactly as before, with brighter lights, easier starts, and wipers that work when the headlights and fan are on.