I know on our van we had in Aus, that was an LPG/Carby set up, that it worked best on LPG or 98 (well actually it worked best on 98, and we used the LPG as a giant jerry can) Fuel cost per km was the same if you were on 91 or 98, so you got a better range on 98, so we tried to run it on that as much as possible. As someone else has mentioned LPG burns dry, so you lose a bit of lubrication; if you run mainly on LPG try doing 2:1 LPG

etrol, so that the engine still gets some lubrication. Also LPG burns hotter (IIRC), so make sure that the cooling system is upto speed.
I'm not sure if that was because of how poor 95 or 91 is out there, but LPG and 98 are closer in their octane rating (I think), so the timing can be set better for the 2 fuels, otherwise there is a big difference - 1 or 2 degrees from memory when we put the timing gun on it.
If I was going to do LPG again I would go with:
-Switchable ECU Maps (LPG/95/91) - ECU's take about 500miles to pick up on a change of fuel grade, they don't just change automatically the timing etc.
-Better system than putting the gas into the intake manifold and hoping!
-2 under vehicle fuel tanks (petrol + LPG), and a rear mounted spare wheel. Have a look at the narrow tanks that can run along the length of the vehicle as well if you don't want to lose clearance
-System that starts on petrol and then automatically switches to LPG
Hope that helps
Please note this all based on my understanding and experience; I'm not sure how right or wrong I am!