i think i would personally only ever fit LPG to a Omega estate.
Because you can fit a doughnut in the spare wheel, and not a huge tank taking up half the boot in a saloon.
You loose a lot of boot space, and means you cannot put the seats down for large loads, which i have done quite often.
Estate, you get the benifits of LPG with none of the draw backs.
I've given this a lot of thought over the last few months and come to the conclusion that a spare wheel tank in either a saloon or estate is too small to give sensible range on a V6.
It worked out nicely on Marie's 2.0 but you'd be looking at well under 200 miles range with a V6. If you're doing enough mileage to justify LPG that's going to be a pain in the neck.
My boot is empty 99.9% of the time, as are the back seats if I'm pushed for space. Granted that's different if your family circumstances differ to mine but I'll be going for a cylinder.
I'd say the payback is so quick at the moment that unless the car's on its' last legs a DIY conversion is always worthwhile. It's a different set of figures for a decent professional fit though...
Kevin