The rear-end in my Opel GT went out once and I upgraded to a Kadet rear end.
The reason? In Europe, the GT came with a rear anti-sway bar. In the US they didn’t so they plugged the four mounting holes in the frame with nylon “screws”. The Kadet used the same rear axle and NEEDED the anti sway bar in the US. So I bought the axle with the sway bar attached (no extra charge), removed the plugs and attached it. It was “sweet”!
I remember the GT.
It resembled a small Corvette.
Someone said they planned to drop a Chevy 350 in an Opel like your’s.
Don’t know how it turned out.