It depends what you want out of it.
I measured a Sabre space frame and a MK V monocoque. I had to cultivate relationships for a while before they would let me measure. And had to agree it was only for one car. There's stuff out there, you have to make private contact because of copyright issues. I think one off type stuff is ok, it's when someone wants to build cars to sell that it gets touchy.
For me it was certainly not cost effective if you consider your time to be worth more than about $5 an hour. But if all you have is time....
What do you want out of it? I don't think very many of the space frames have much torsional rigidity. And it takes a lot to design it into them due to the shape of the doors. But if you just want a fun street-er maybe track day car the basic space frame that started with the KVA is pretty simple and not much has changed among the different people that have done/ are doing it still.
I scratch built a Cobra chassis but the GT40 is way more difficult (4X?), much more crowded and the mid engine adds complexity and cost. There are differences among bodies also so you probably will have to tweak dimensions of your chassis for a proper fit.
The Cobra was a Shell Valley and they sold me some rudimentary plans for $50 which I improved upon quite a bit including a Jag rear.
It may pay to do something easier first, but it's still possible with resources like this forum, you are jumping into the deep end though.