I looked into this some time ago and thought that it would be pretty easy and a inexpensive thing to do. Sorry I don't remember the detais but I found that in Texas the regulatory requirements turned it into a $3000-5000 dollar investment. If the kit did not have the EPA seal of approval you couldn't get a state inspection. I also think they changed the EPA requirements for 2007 making it more expensive. In my city there were few refueling point for non commercial cars.
I don't know exactly what you're looking for but here's a basic link that might get you started.
Thanks for the link, I'll get reading on it right now. I would prolly just disassemble every two years for inspection,
I wonder if it could be run purely from a supplementary point of view...
Yes I think it can but here it would still need Approval. They make small tanks(10 gallon I think) that are shaped like your spare so that you can remove the spare and put the tank in there. Although with 10 gallons you're not going very far. The tank is the biggest expense.
Keep me posted on what you find out I would interested in know how cheaply it could be done it you did it yourself and difficulty of installation.
a few years back I checked out a book published by SAE (socity of automotove engeners) about LPG and CNG in cars, it was about the instalation, and matenice, might be worth checking out what has been written in paper form.