It might be easier now MMC use Mazda gearboxes....at least for the 4/4 and Plus4 as there is an auto option for the MX5 and I believe the gearbox dimensions are similar to the manual box. There will be an auto box option in the 3.7 Mustang but that could be a bit bulky for the footwell width in the Roadster.
Two out there.....well, one in the Morgan museum, Jane Morgan's Plus8 engined auto Drop Head Coupe.