Hi Kellysguy,
You don't want to go with a 1275/OD combination for reasons of originality...
...but you want to put an overdrive in a car that never had one, and you don't want to use the only OD that the Midget / Spitfire 1500 engine came with?
I'm not trying to be obtuse, but I'm not sure that I understand the question. If you're asking 'is there an overdrive or OD kit that will mount to a 1500 engine, and will require no body modifications whatsoever to the Midget transmission tunnel or crossmember?' then I think the answer is no. Not that has been developed / marketed, AFAIK.
I'm pondering the same thing, but planning on the Laycock OD. Rust-free California car here as well, but I'm willing to grit my teeth and do a little cutting, for the sake of increased drivability. Work to be done next winter, I think.
Best,
Duncan