For V6 engine it is not worth it. I have the installation in my ST220, which i do not use. There are couple of issues with it. Specifically my system is Tartarini (there are also Prince, BRC, Lovato etc.).
First issue is that it is extrimely hard to tune LPG system for the V6 (or any 6 cilinder engine). It never works quite properly. In my case, i had an explosion in the inlet manifold while idling because the mixture was not tuned properly. thankfully only a blown hose was the result of the explosion.
Second issue is that fords v6 enigne needs lubricator while using LPG. It uses an oil canister installed in the engine bay, which needs to be filled every so often. The issue i had with the lubrication system is that the control valve on that system broke and released all of the container contents into the first cilinder, whch resulted in a burned piston, broken cylinder etc; basically engine was trashed, and i had to put a new one in.
Idle on LPG and V6 engine cannot be tuned properly, which means that 10% of time while pulling up to the red light or stopping your engine will cut out, because the RPM will drop rapidely and Idle control valve cannot respond fast enough and engine stops.
On other hand it is more economical way to drive and it is enviromentally friendly as mostly water comes out of the exaust (be prepared that your exaust will rust)
In my opinion V6 engine is not meant for economy runs, or saving petrol. thats why nice engeneers of ford have come up with ecoboost 1.0 engines