You are correct on the US. The one to go for would be the Camero 2010-2011 312hp, 278lbs-ft LLT or 2012-> 323hp, 278lb-ft
Which would give you about 50% more over standard and I would expect a much lower first gear with a 5-speed autobox.
There are also 304hp 273lb-ft LLC fitted to the Cadillac STS from 2008-2011, which maybe the best option when looking for a donor write off at a reasonable price.
Fuel consumption is reasonable:
http://www.car-emissions.com/cars/view/28301http://en.wikipedia.org/wiki/GM_High_Feature_engineMy thinking is what can you do for more power and still get around current 3.0 Miggy fuel consumption. With the 3.6 Extra-Urban fuel consumption of 37.7mpg, which I suspect would be around 30-33 in real world condition, you would be looking at 45-50mpg costs with LPG.

Of course we would all like to put a Vette V8 in our Miggy but I suspect for many the runny costs would not make it very practical.

If 50% more power and 40% more torque with a 3.6 V6 is not worth it, then best you stick to your F1 engine plan.
