You should really use a Woodward Model N series regulator/vaporizer. It only allows propane to flow into the engine when there is a very slight negative pressure on the the output side of the regulator. This will gaurantee that no propane leaks into the engine when its not running.
Use that with a Hobbs normally open pressure switch, PN 76573 4NO. It comes set @4psi, but is adjustable from 2-6 psi.
You also can not use the typical propane tank fitting like a grill has to connect the tank, Theres a special fitting that screws into a BBQ tank so the system will work properley, I forgot the name of it...oops. You can get it @ Homedepot though.
I will post a few pics of my entire setup this weekend for a better idea. I made a quickie diagram of my setup for you to see and get an idea.
I use my setup alot!! It works great with no problems yet.
If you have any questions Ill be happy to try to help.
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04 QC/LWB 48RE 4x4: EJA, Arson CP3, BHAF, 4" TB stainless steel exhaust, 7" stack, Propane injection, Swanker intake manifold, GDP heater grid delete, BD 100 gal/hr pump, Big line kit, spin on fuel filter, Racor seperator, FS2500, mystery switch, 5 gallon on-board air system, Viair comp, Optima blue tops, 1875 Watt dvd stereo system, train horns. 6" lift w/35x12.50/20's