Hi. Do you still have any GT stock radio?The (AM) radios are all pretty much the same, but there are at least 2 different face plates. The early ones all said "Kadett Radio", even on the GT's and the later ones said something different. I have at least 6 of them that were removed at the dealership from cars who's purchasers didn't want to pay the extra for the radio. Not quite NOS, but pretty sure they'll work fine.
I also have 3 or 4 Datsun and Toyota AM/FM radios from the early '80's. They fit in the stock holes without modifying anything, look pretty close to the same, but let you listen to FM. One of the Toyota ones has a remote mount tape deck too, but I can't remember how well the tape unit worked. Another has a remote mount 8-track.....