H-Approval how original does everything have to be??

Hello together, I have recently seen a show about the restoration of a Ford Mustang, where the one who built the car together with the Tüv on the basis of old brochures etc. has proven the original condition of his oldtimer for the H approval… To how far has one here actually room for action regarding safety… My consideration: build an old Ami (Mustang/Camaro or similar car from the 60s). But I would feel more comfortable if I had such a car. it could equip 3-point belts (also in the interior) and possibly a better/moderner braking system… then the vehicle still suffices the conditions for an H-approval? Does someone know each other in the things and can write something about it?? thank you in advance !!!!