The GT40s spare wheel was carried in the front, under the removable panel, behind the radiator outlet. Thats why the outlet was usually two triangles, as the wheel was in the middle. It was the luggage compartment that was next to the exhausts. This was originally a large biscuit tin, about a foot square, which happened to be the right size.
I thought that the 917 had a spacesaver spare between the exhausts. At one race, they lost the spare and found that a front wheel wouldn't fit, and they couldn't even get it in the cockpit.
Apparently, the spare tyre/boot rule lasted till the 1969 rule change. I would have given it another few years