At risk of being boring etc, my tuned V12 had ex 3.6 getrag five speed kit fitted properly and it has never caused problems, see posts on other Jaguar forum.
Needed full kit from 3.6 ie pedal box, good gearbox etc, plus adapter plate with mods for V12 starter. Machined at least 12lbs off flywheel and clutch is the key, cant remember what we fitted but you need an excellent clutch re torque and it properly set up to be light enough in town use.
Also had to take about 4" from propshaft and have it carefully balanced. Also fitted different rear axle ratio, it is a quick car and slightly more economical on out of town/motorway use.
Well worth the effort and cost.
Supra boxes can also be done.
American boxes are expensive but may well do the task.
I also have a 1985 V12 fully converted that could be for sale, my 1983 is not for sale.
Keith