I echo pvr comment re LPG. Be very sure before going that route.
I've run it for years on a 6litre Jeep and will never break even, but I did it for other reasons (better than carbs off road)
Tank can only be 90% filled.
It's lower mpg than petrol and drops power a small amount.
Loss of luggage space, frequent stops, slow to fill, etc.
If you do big mileage and keep the car a good while then it works.