have you checked with gotpropane.com??
Easy enough to put your own together, though. I just did a 258 from parts and pieces. Major parts came thru the FS section here, but originally came from Nash Lift Truck and the Propane Guy (propaneguy.com). Fuel line and fittings I picked up OTC in Raleigh.
Hardest part may or may not be finding or machining an adaptor from the throttle body to your intake manifold.