The Isuzu housing is about 4" wider than the GT housing. You'd need substantial flares to fit it to the car, or you'd have to use front-wheel drive offset wheels.
As far as cross-referencing parts from other Opels.....no, they're not easily obtainable nor cheap. A limited slip (ZF) would set you back about $1200 or so, a GM bearing and shim set another $400, plus the labor to install it. And then if you decide to change gear ratios, another $300-$900 depending on the ratio. And then you STILL have the weak torque tube, which requires at LEAST an overhaul of all the parts if it's gonna take the abuse of an engine with more torque. New 'donut', new side mounts, new top/bottom bumpers, possible new T/T bearing. More $$$.
A swap is more labor intensive, but far cheaper in the end. Travis went this route for his, GT, narrowing a Toyota truck rear axle, but he also was able to pick up a limited slip and 4.88 gears for a LOT less money than anything Opel.....and it's a ton stronger. I mean, they put Small Block Chevys into Toyota 4 X 4's, and even with the added weight of a truck plus V8 (3300 lbs +), they hold up to most normal use.
Bob