I am an LPG convert. I have DIY fitted LPG to two cars now, a X300 Jag, and then I fitted the same system (with a few extra bits) to my 4.6 V8 P38.
Both cars have run very well on LPG, the P38 gives me about 235 miles to 74 litres of gas or so.
It's dead easy to fit yourself, and set up. So long as you can use a Meccano set and operate an electric drill, you can LPG your car.
There are always loads of stories about Rover V8s slipping liners on LPG, but theres no evidence. They slip liners in TVRs and in countries where LPG is almost unheard of like the USA. Just because a V8 slips a liner on LPG doesn't mean the LPG caused it to slip the liner.
Fact is the Rover V8 at 4.6 litres is not a strong engine, no matter what you run it on you might get unlucky.
If your engine is fuel injected get a recent sequential LPG system, and it'll run great. The LPG system copies the petrol injection pulses so there no need to "tune" it for petrol or gas. As you drive more and more on LPG the engine management will use the feedback it gets from the engine to continually tune the engine just like it does on petrol. They even set themselves up automatically these days.
Mine's been great for nearly two years now, so far no problems, just reasonable running costs for a massive car with a thirsty engine.
I'd say go for it, with the 12,000 mile oil service periods the V8 has, I think it makes running one actually cheaper than the diesel.