Thx for the nice welcome.
Monte is ready again, just got new radiator, coolant hoses and the heater thing for the inside (the thing that likes to leak into the passenger side

Yesterday I kind of scored big time as I found a NOS Delco Radio, still in the original packaging for 50 Euros, which is significantly cheaper than repairing the tape deck of my old system :))
@Pit: 87 bumper ???, where's the difference to a 86 one ?
I meanwhile know that this car must have been directly ordered for a customer by a German dealer, this model was never intended to be sold officially in Germany,
But that's it. Opel/GM Germany can't and won't help anymore as they don't deal with anything Chevrolet anymore.
Will pass soon by a very long time and well known US-Cars Dealer here in Munich, maybe he sold it in 1986.