Propane injection question?

i have an old dodge pickup with a SB 360, can i put some kind of propane injection system on it? if so what modifications do i need to do with my truck, can i run both gasoline and propane, or just one or the other, i only ask because my pickup gets about 7 mpg and propane costs like 1/3 of gas so any info on propane injection, prices mods parts ect. would be a big help, thanx in advance.