First, you'll devalue the 780 by about 99% by converting it i would think and second, while it's a viable proposition, you'll need a left hand drive pedal box, clutch hydraulics, M47 gearbox, lever, interior gear lever surround including the rubber seal, clutch assembly, manual gearbox loom (for the reverse light switch and inhibitor switch link-out) and a lot of time and patience.
There may be other parts i've missed, it's the best part of 30 years since i did some auto-manual conversions and they were mainly SD1 V8s and Granada V6s although i did some others i can't remember now.
You may also find that if your current auto-box is electronic, the engine doesn't run right because it's not getting feedback from the auto-box ECU, particularly under boost conditions.
Personally i'd leave it as is and enjoy it!
