Bela,
Which model number or a picture would help. There are several coupe owners who have Beckers of the early 70's type including the seperate cassette in the glove box. But the Grand Prix went through several changes, for instance Mercedes with a full face DIN; rather than the earlier versions.
That type radio would be difficult to install.
The biggest issue seems to me is the depth with the a/c if you have that, the angle of the face plate, trans hump, and how may connections for external amps, etc you're dealing with. It can get crowded in a hurry whereby the radio won't sit flush. I usally take off a side panel to make sure everything is as compact as possible.
I'm sure someone here on the board can help you as I now use an Alpine head but could easily change back to a period type as I saved my Nakamichi. But I'm sure something new might catch my eye as it constantly changes even now.
There are several posts on Beckers found by a simple search.