What's wrong with your current seats? If you still have some, the best thing you can do is sigh, gasp, and crack open your wallet and redo those.
If they are missing....
Miata seats, you have to reduce the height of the seat base, this has been done. There is a couple of fella's on the NASS site that have done that successfully, and the mounts need to be modified to fit as well.
There are rumours afoot, that the current hot set up is Porsche 914 seats, but I have not sat in any Spitfire that has had a set of those in it, so I cannot speak to that.
I'm in the same boat, It's time to redo the interior of Jehu, and I will spend as much on that as I have in the rest of the project.
NOTHING else fits. Figures.