Triumph Repair: Spitfire 1500 J type overdrive, spitfire 1500, speed gearbox

Question
I want to put a J type overdrive transmission into my 76 MG Midget. I am told that the Spitfire Transmission will bolt right up to the engine because as you probably know the late model Midget uses a Spitfire 1500 engine and non overdrive transmission.
Here is my question:
I have located a J type O/D unit without the transmission. How difficult is it to install a J type O/D unit to a transmission (gearbox)? Do I need a specific gearbox or can I use my existing transmission and install the O/D unit to it? Also if the O/D can be installed on my current transmission could you tell me what parts and bits i would need. Any help would be greatly appreciated.
Thank you

Answer
Hi Louis,
You can not just bolt an OD to a non-OD box even though the case bolt pattern is the same. The gearbox mainshaft is different. You would need to find a OD gearbox mainshaft and put that in your box. You may have to do some body work to the transmission tunnel as it may not accept an OD. Also you may have to get a driveshaft shop change the length of your driveshaft. It will be easy to wire in a switch to operate it but you should tap in to the back-up light circuit and add a relay to prevent engagement of the OD in reverse as that can destroy an OD.
It will probably be easier to find a 5 speed gearbox and make an adapter plate and change the clutch system and driveshaft to match it all up. You may try John's Cars in TX as they have several kits to put different transmissions in different cars.
Howard