I've contacted vintage wheel a couple of times via email for the same thing but no response.
http://www.vintagewheelsus.com/ They have 17" cobra knockoffs.
Other possible contacts are:
http://www.team3wheels.com/ac/ They have bolt on wheels for cobras.
PS Engineering also has a cobra knock off and does custom, but I don't have any contact information for Phil Schmidt (the PS in PSE)
I'm a little worried about a knockoff adapter adding width to the front wheels. It would use up what little backspace there is, and I think you'd lose the front lip. A bolt-on wheel with a cover over the lug bolts (like team3) would allow the widest front lip.
My other concern is will the proportions look right on an 18" or 19" wheel on the GT. The GT40 wheels were originally a 15" design, correct? The 17" that vintage wheels sells still look great to me, but I'd like to see a picture of 18 or 19" versions before I buy.
Keep us posted, cause I'm interested in the same thing.