I pieced mine together with parts from a local wrecking yard, and bought a 320litre tank- used for a bottle of scotch, heh(it was even half full!). That was a few years ago now. I researched the "rules" and mounted the tank, lines, and components as per the regulations where I live. I had a butterfly window from a donor truck with the lpg sticker on it, so I mounted it, and have never had problems getting fuel-here. One day, I will get it inspected, pay the $60 some odd dollars for the little sticker, and it will be legit totally. Nowadays, I would never pay the 2grand plus for a new install-that could buy alot of gas! However... If you have a truck with lpg installed, and needs a new engine-then build it strictly for propane. Recurve the dizzy, choose a decent but not wild cam, and keep the compression to 10-1, or 11-1 depending on loads the engine may see. There are alot of other things to tweak the engine for, cept I don't have time to type it all out.
I'm at a dilemma now, my 429 is getting tired, had to put on some D3 smogger heads, which dropped the compression somewhere in the low 9-1 area(compared to the 69, 11-1) and sure makes less power than it used to. Now I either should rebuild the 429, or build an 11-1 460ci, or try something smaller, like a 351 with longer rods, or even a 393... High fuel prices and hot v-8's sure takes the fun out of it. Propane is 85.9c/litre and gas is 124.9 c/litre here, and price per barrel just jumped up again on the news.
Ohya, I'm a first year auto tech, and my best friend is a journeyman mechanic-so please don't try to install propane yourself if you don't know much about it.

 

Here mine is without the air cleaner.

 

I know from working on airplanes that many of the lifts we used usually started on gas and then we switched to propane. If you have any lift companies in your area you could ask some of there people they might have a lot of experience in that area as well.

 

They wrote about an install in 4 Wheel & Offroad a few months ago. Here's the link:

Dual-Fuel Propane System Install - 4Wheel & Off-Road Magazine

I'm thinking about using this system on my 78 when I get the time and money. But I'm not going to do it for any of the so called benefits. I just want the cool points.