Sabot, regarding Spitfires/GT6s, the seat may fit the car, but will the seat fit you?
The Spit seat really hugs the floor pans, it's a very low design. Depending on your height and inseam length, other, non-stock seats will render your seated position too high. Or, if you're, ummm, a wide person the Mazda seat may cause your leg to cramp very uncomfortably against the gearbox tunnel.
Don't make the mistake of purchasing "foreign" seats for your Spit or GT6 without careful research. They may not fit your body type.
FWIW, YMMV.