Hi Pierre,
As you are finding out, when you line up two cars that are claimed to be painted Viking blue, there can be significant differences. If you look at the pictures of the Falcon and the F100 shown here....
http://paintref.com/cgi-bin/paintdet...%20Blue%7C8906
....you'll see some marked differences in Viking Blue. Also can you rely on the various paint suppliers' interpretation of the original early '60s Ford paint code?
If not, I would suggest that when you find a Cobra 289 painted in a Viking Blue interpretation that you like, speak to the owner and take note of his car's paint code.
Alternatively, could do as mentioned previously and find a late model car that looks like a match to what you think Viking Blue looks like, obtain its paint code (from the owner or the manufacturer) and go with that. If you're fortunate like I was, you can potentially see the car's colour in the shade, in bright sunlight, under fluoros etc. and know (approximately) what your car will look like under those conditions. Last thing you want to do is pick a code, have the car painted and be disappointed.
Good luck!
Cheers,
Glen