Don't have much experience with dual fuel set ups as all the vehicles with LPG I have owned have been straight gas set ups. The big limitation I have found has been the distance you can travel on a tank of LPG and the avaliblity of LPG in Western parts of NSW. Think west of Bathurst. I had an LPG system in a HX parts van I had but didn't keep the LPG system for the replica van build as it will be a weekend car so fuel bill should be bearable.
I am wondering if you can buy ignition only computers that allow you to have two igition timing curves so you can have a LPG and petrol setting?
The other issue I have found with LPG is the cost of the stuff when you get further and further from a capital city. The price of LPG seems to rise at a much stepper rate than petrol has been my observation.

Anthony