Triumph Repair: Spitfire J type overdrive, drive shaft, driveshafts

Question
I've purchased a 2nd 1976 Spitfire with overdrive and intend to install the overdrive on my other 1976 Spitfire. First I'd like to know if it will be a direct swap or will I have to modify or change drive shaft coupling? There is a switch mounted on top of the transmission cover that I assume is the reverse lockout, but the books don't refer to it. Am I correct? There is not a wealth of information on overdrive rebuilds. Are there any particular cautions I should be aware of as I dismantle the unit for inspection?

Answer
Hi Rod,
Unlike some cars I think the driveshafts were the same but that don't matter because you will have both. As I remember most everything is a bolt-in with no modifycations necessary other than you will have to add the wiring. As I remember that too may even be an add on by just removing the OD harness from the OD car. It is needed to block out 1st, 2nd and reverse. Especially reverse because the OD will self destruct if forced to go in reverse.
As for dismantling to inspect. You should get a service manual before doing so as a few things inside are not self evident.
Howard