Well, this is where I have to be careful because it is my own compmnay which can do this thing. So the following is by way of engineering explanation, not promotion.
Firstly, talking generally about using diesels inplace of petrols. At present we have a range of conversions for Land Rover Defenders, including 4.2 Supercharged Petrol V8 (460 bhp, 570 Nm), 4.4 Normal Petrol V8 (320 bhp, 422 Nm), and 2.7 Twin-Trubo Diesel (240 bhp, 530 Nm). It is torque which breaks transmissions (together with very bumpy roads/paths), so for the 4.2 SC and the 2.7 TD, we uprate the propshafts, diffs and halfshafts. We have often been asked to fit the 4.4 litre Land Rover TDV8 engine but there is no way it can be worth it becasue the transmission and chassis just can't deal with 800 Nm. Similarly, the 460bhp of the 4.2 SC is rather challenging, but at least it doesn't break anything, by itself.
The point I'd like to make is that when retro fitting an engine to an older car, it is the torque figures one must look out for, so in turbo-diesel terms one tends to end up with a smaller capacity engine, for obvious reasons. As I understand it the 411 engines made about 450 Nm (335 lbf.ft), and we can easily exceed 500 Nm from the 2.7 V6 diesel, so it is more than enough. The 240 bhp is also plenty to get a Bristol to cruise at well over 100 mph, if not as fast as with the 6.3 litre Chrysler engine. The main difficulty is persuading a hydraulically controlled gearbox to changeup before the diesel rev-limiter comes in.
As has been implied, these modern diesel engines tend to be tied in with loads of electronics and the electronics of the auto gearbox. We buy in an aftermarket, stand-alone ECU to run then engine and we similarly have our own controller for a 4-speed ZF gearbox. Trying to modify the original ECUs for these engines and gearboxes is virtually impossible unless you have full cooperation from the manufacturer, and Bosch, and ZF, which aint gonna happen.
For the Bristol, the most practical solution would probably be the V6 diesel with the 4-speed ZF (it has a lock-up TC), and keep or sell the old engine and box together.
Also, because of all the electronic and wiring loom costs, these conversions are no longer cheap and cheerful. We use new engines, but even with second-hand units I doubt you would get much change from £10,000.
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