I am working on this exact project.
A golden cuckoo is a barred black copper, wheaten or gold duckwing (rarely exist) marans.
The best and correct GCMs are barred black coppers (aka gold birchen).
You need to introduce the barring gene (B) to the birchen pattern (Er) while retaining pure gold birds (s+/s+).
The barring can come from a dark cuckoo marans or a silver cuckoo marans. Silver will be easier but they are much rarer. Many ‘silver cuckoos’ are mislabelled Dark Cuckoos.
And the birchen and gold will come from black coppers.
You need both to breed GCMs from scratch.
I am planning a couple of breeding programmes to choose the optimum one, I will update you with these in a while.
How many breeding pens and birds would you be willing to have, so I can send the most ideal breeding programme to you?
hope this is helpful, feel free to ask more questions.