I have a old F150 with a 351m and it has a propane tank that it ran on back in the day but now it only runs gas and I want to run it back to propane but I'm not sure were to start.

 

I would start first by asking the local propane supplier if the tank itself is still legal. If they regularly sell to vehicle fleets, they may be able to inspect the whole system for you.

 

We don't really have a place like that here but the tank is holding propane and works but I will get it checked when I bring my truck to a larger town. What would the next step be?

 

Inspect the tank, No deep rust pits, No deep gouges or dents and the Data tag HAS to attached and Readable, If not its not scrap. Also if the relief valve is more than 10 years old it should be replaced. As for the rest is there any previous equipment left?
You will need hoses, a LPG filter/lockoff, Gasoline lockoff, LPG vaporiser, LPG mixer and nessisary mounting hardware. You need to work with a local LPG supplier to procure this equipment and also buy fuel. Also one other item I would not even consider converting it without a full check out of your motor compression test, cylinder leakage, oil pressure etc. Unless you plan on rebuilding you motor and starting fresh.
Good Luck