Adam, I agree with you regarding installing the front two bolt holes. They were angled back but only at an angle that matches the windscreen on the original donor vehicle, this was certainly a lot steeper in rake than found on the GT40. I presume the GT40 dash would be easier to remove than the donor dash, so it might be possible to install the dash vent pre dash install. Again, not sure how this all worked in the MK 1s and what was underneath the vents. I'm of the generation that was born well after the GT40s had finished racing and just enjoy researching and learning about them.
Cheers.
Ryan