Over here in the Netherlands almost all V8's are running on LPG/Propane. Primarily because the price difference is around $4,50 a gallon. Yes you heard it right, difference is $4,50 a gallon
Anyway, I think 90 percent of the older LPG cars are running an Impco system. 300a's and 425's in combination with a model E regulator. Most dual fuel, some single fuel.
My experience is that well tuned LPG system with the right parts can do a great job without making concessions in power and just a little in MPG. Impco 425 single fuel is the best way to go. Make sure your timing is set right, use the proper sparkplugs, get cold air flow to your mixer.
Single fuel makes it also possible to take your gas system out. Tank, pump (leaking in you engine), carb, lines.. This way the weight of your vehicle stays the same..