You can call MHS ( Material Handling Supply) at 562-921-7715. They are a Mitsi / Nissan dealer in Santa Fe Springs.
They can set you up with a aftermarket carb kit and all the other needed parts. When you call make sure you have the trucks Make, model & serial number, it helps them a lot. Whan you set the motor up for straight LP make sure to replace the spark plugs and gap them to.28-.32, I'd just do a major tune up since you'll be so deep into it anyway.