I agree, it's a feasible swap to do, but MAN those suckers are a lot heavier than the Opel rear end and bigger too. You'll have to cut out the floor of the GT just to fit the center section under the car, unless you raise the ride height. Forget it on a lowered car unless you do some serious cutting.....
a cheaper and easier way to do this is using an axle from an isuzu pup truck
i measured and the axle housing ends are the same width and they are pretty much the same dimensions as you stock opel axle
one thing that would need to be done though is axle redrill and whatever else because the isuzu has a six lug pattern
second, you would need to have a jigg to line up the brackets from the old axles to mount the axle
or another option would be to go ahead and fab your our setup and go coil oer shocks and make ladderlink bars to connect into your stock control arm mounts and still retain the panhard bar.
next would be the drive shaft so i wount be cheap but it would be cheaper than the other axle plus i think it would be and easier application conv. also the next advantage would be the fact that you can get a rearend for abnout $50 from the junk yard slap some seals in it and you have three gear ratios to choose from
a 3.44:1 , 3.73:1 , and 4.11:1 if you would like to know what year/model/engine had which ratio email me at [email protected]