Trying to find the gasket that goes between mudguard and rear light but with no luck, cannot see one in the parts nook either!!
It's only a bit of rubber beading with a bit cut out for the mounting bracket (at least is on mine but may well not be original).
As far as I know there weren't any gaskets used on any of the pre-oif unit single rear light housings (1 type used on 1967 Barracuda (Starfire for US) then another type for 1968 that covered the rear support rail and another shorter type (didn't cover the rear support rail) for 1969 and 1970 Starfire. HTH
Don't know if this helps