I have owned/raced Datsun 240z which don't have a lot of room for a full cage. Your head is very close to roof/door bar and depending on the cage installation and your height I have seen drivers helmets rub on that bar during a race let alone an impact. I have owned/raced a classis mini also these are a small car but have more room that a Datsun 240z for a full cage...but again it is still very close to the occupants.
A GT40 has much less room that the 2 vehicle mentioned above. When I start my GT40 build it will NOT be getting a cage or any exposed metal structure inside the occupants area. Especially after reading the pdf Frank shared and other comments/feedback in the thread.
I consider driving these older designed vehicles much like riding a motorcycle on the road. Not ideal to have a crash in period.
Modern cars have so much safety features i believe it is robbing us of correct driver education/skill set. People almost think they will be ok in an accident due to all these safety features. But the safest way to avoid injury is drive to the conditions/cars/drivers ability and avoid a crash in the first place.
Be safe out there and respect other road users they have families too and want to get home to them.
Yes at times I ride my motorbike like a loon when the conditions are right
but I also ride daily so know the risks I am putting my self in.