[alffie]

most V8s will get that sort of mileage sitting on the highway on a 110 in top gear. Your cobra will easily get 10L/100 on the highway CRUISING. push them hard and it will be more like 25L/100.
If you build any engine to run only LPG and you will find they will more than likely make more power. Like mike said the LPG has less BTU so you will use more but LPG is half the price of ULP. The only problem will be where you can fit the tank. Talk to Vic Roads and see if they will let you run a windsor on LPG.