My Omega (3.0 MV6 manual) is dual-fuel, if you have a filling station near by that sells it then I'd consider it a way better solution than a diesel. Loads more power (210 BHP vs 130), much smoother, more fun as it revs to 6,750 RPM, and almost certainly cheaper on fuel. My trip computer is showing around 27 MPG average, that's over the last few months since it was last reset. Take into account that propane is less than half the price of diesel (we'll be conservative and say half price), you'd need to average 54 MPG in your diesel to get the same economy cost-wise. Not gonna happen!
Admittedly the economy is a bit higher than normal cos my dad's been using it for the last month or so as I've been using my BMW (I live in the city and only have one parking space, so whatever car I'm not using gets left at my folks' house) so it tends to be driven a little more sedately than I would, although my dad certainly doesn't drive like miss daisy so it will be wound up now and then. Even then the lowest I've seen the average MPG display is around 23-24, still loads cheaper than a diesel.
Your best bet in my opinion is to look for one pre-converted. They're only a little more expensive than standard V6s, which is a huge saving over buying a V6 and converting it yourself. Just make sure you give it a good test drive to ensure it drives and idles well on both petrol and gas. A good installation will be barely noticable when driving, it should feel just like running on petrol (a very slight power loss is normal).
Good luck!