I'd like to be able to get more than a couple hundred yards, so I think I'll clamp something to the luggage rack.
MFJ Enterprises part #MFJ-342T or any "trucker style" mirror mount with a 3/8x24 threaded stud should work as a luggage rack mount. You'll just have to experiment with the grounding of the luggage rack to the car body. Maybe the existing bolts to the trunk lid will be enough, maybe you need ground straps, or capacitive coupling, or inductive coupling, etc.
I gave brief consideration to something like
[this] or
[this] but I think the body would need to be drilled out,
Probably. The first link is to a "gumdrop" style mount which needs a 5/8 inch hole and a lot of room around the hole for the gumdrop/dome; they're designed as roof mounts, not top of the fender mounts. The other antenna you link to is manufactured explicitly as a drop-in replacement for a Honda Goldwing motorcycle OEM FM broadcast antenna from the mid-1980s. It would probably never fit a Spitfire.
and then I'd need some sort of wedge-shaped spacer to get the angle right to get the antenna vertical.
Nah, you use something like a Workman C406 or ProComm JBC100 swivel mount to make the antenna vertical. Spin the mounting stud to get the right azimuth and tighten the stud, then the 180 deg. swivel extension gets you to vertical. So spinning the mounting stud gives you one axis of movement, the swivel adapter a second axis, then you can get to vertical on most vehicles with just these two axes of variation.
