Volvo LPG Conversions - What is the best car and system?

[SteveV40]

Hello all... My trusty 2002 V40 1.8 SE BiFuel which I got at 60k has now passed 250k, and is still going strong. To extend its life I am thinking of semi-retiring it but I have a dilema over whether to jump to diesel (gulp), or to look at a more modern petrol vehicle 2-5 years old, and have it LPG converted since Volvo have stopped the BiFuel models. I currently get a real all in average fuel cost of 14.1 pence per mile, based upon 4 months careful recording over a distance in excess of 12,000 miles (mainly out of town and on motorways) and filling the petrol tank when doing the main calculation. That gives me an equivalent of almost 45MPG, based upon fuel cost. My last full fill to full fill delivered a cost per mile of 12.69 pence, although it is warmer now and both fuels have been cheaper recently. To match the 14.1ppm average, I would need a diesel to give me 45mpg on the road (not "official" figures) and I am looking for an improvement to 60+ really.
So.... Does anyone have any thoughts on what petrol Volvo would give me a long life and a 60+ MPG (equivalent) with an LPG conversion? And what conversion would be the best? I do about 35k+ per year, so the base costs are not the real issue, but running costs are. I have to say, my BiFuel has been fantastic. Any help would be very much appreciated. Many thanks. Steve.