We've done the drive via Plymouth - Santander ferry, right down to south-eastern Spain than back across to Faro and more westward. Done that three times but not with the Disco, always with the company vans and our trailer.
Regarding the 3.9, we have had ours a while now and it has settled down to a reasonable co-existence, 18-20mpg toodling round, 14-16 on motorways, about the same on gas.
Gas is a must if petrol is at UK prices, I'd love to take it over to the 'States and run around over there on their cheap petrol!
As Discool says, the gas fitting and type of system will make or break the conversion, the best ones are the later injection systems where the engine has petrol injection to start with, rather than carbs.
Ours is a 1999 Thor injected V8 with Zavoli multipoint gas. We have twin side tanks which give us about 150 miles running out to the field to feed the nags or general motoring, on the trip to Scotland a few weeks ago we averaged a surprising 19mpg on both fuels, but then on a trip you are steady at one speed with little full-throttle work.
Compared with a diesel, I found the V8 fiddly to drive until I got used to it, but our Renault Trafic van with the 150hp engine will show it a clean pair of heels on almost any driving, mainly as it is nearly a ton lighter!
Peter