Your boot floor looks very similar to my set up

It's a complete new panel to replace the wheel well and allow fitment of a fuel cell

Does it have a sunroof? If not then it's a motorsport shell, to find out if it was grp n or a like you said you need to know the previous spec.
To me if it had solid mounts etc it says group a. As grp n were pritty much road cars
Has it had a paint job? What colour is it underneath?
regards Aaron