Why is a Radical and a GTD different? They start as simple tube-frame chassis with suspension and a fiberglass shell.
The GT40 is more complex in that it has windows and doors and cladding etc...but it's not a different animal at all...GT40 was first and foremost a race car and was altered to be a (barely) street legal car. If I'm not mistaken, this is the appeal of the car to most of us. Unfortunately the US market has special needs as regards to putting the car in harms way on race tracks which limits the appeal of the current kits. Most of the rest of the world doesn't have this particular sort of restriction.
In this configuration it could be legal for SCCA Trans Am, SCCA GT1, NASA ES and NASA PS1 and OPEN TRACK CHALLENGE. I don't see that as particularly limiting as most production cars are eligible for one class only.
Here's an Ultima GTR:
And here's a GT40 tube frame:
The crucial difference is lack of a roll cage with appropriate support. Both cars have fuel storage problems and I'm still trying to figure out how an Ultima can be an SCCA and Grand Am legal race car given the dicey fuel storage...there must be a modification they do that might help the GT40 community.
GTD isn't going to come up with the support since they're out of business so hopefully one of the other enthusiastic manufacturers will. I believe one of them is on the right track and my rant is not against any manufacturer but a call to move these kits up one logical step so they have wider appeal and utility. A GTD with a cage and modern fuel storage and management is a safer road car too.
Pricing is an entirely different issue but remember that race cars have vastly fewer parts than "street cars" and given things like seats and belts and wheels and brakes have to be race spec, they are easily obtainable outside a basic kit so actually simplify what a "roller" is.
Convincing a sanctioning body that your modification to someone elses tube-frame chassis is NOT as easy as it sounds...and it's expensive. Certification from an engineering company is only part of the problem. The designer of the chassis is the person who is most able to safely modify it.
I believe (at least in the US) that several vintage groups now allow replicars in their series. All the more reason to broaden the appeal of the GT40 design to be a car with modern race equipment.