Greetings,
Someone asked me to post some pictures of the Miata seats I recently wacked into my '71, so here goes....
They're very similar in shape to the OEM seats, comfortable, have speakers in the headrests, and are *way* less likely to fry your thighs after a day parked in the Australian sun.
I used the Miata rails, drilling a hole in the front section to match up with the front mount on the floor pan. Then simply drilled 4 holes in the mounts (2 to match the new holes in the rails, 2 to match the existing rear holes), wriggled things about, and bolted them in. I've still got some overhang (the Miata rails are longer than is required) and will cut it off in due course.
Relatively simple exercise and a useful alternative for a DD.