Nissan Patrol

don't know about patrols but this happened to our forklift a few times, it was the valve that cuts off the gas was leaking( the 12volt one for safety).changed that and was fine.

I thought that problem mainly happened in the efi models, I"ve seen people drill a large hole in the side of the box and then plug it with a large cork from the outside so it blows the cork out instead of blowing the box up

It's critical with LPG to have a good healthy spark to ignite the fuel/air mix. There are many thousands of Patrols running around like your friends one with few problems.
There are some things that can be done to help this and they include the correct heat range of spark plugs, a good set of ignition leads (I use Eagle LPG ones on my efi Patrol) an extended rotor button and perhaps the best thing is a good electronic ignition system doing away with the point altogether. I believe the Petronix ignition conversion is highly recommended for Patrol's.
Regards Andrew.

Funny you say that, my 89 gas/petrol patrol went the other way and blew up my $200 muffler on starting yesterday, this is the first time this has happened to me in 4 years.
Wazzaaaa

Can only double up on ignition post, as a lpg installer spark mis very important on any lpg vehicle ,if you install good lpg leads and extend rotor it should be fine Just make sure you dont touch the acc while starting as it will backfire no matter what ignition is in it.....Good luck

had the same issues on a hiace van and a ford ( injected ) ended up just putting a jockey strap over the box to hold to top down and in the event of a backfire the strap would absorb the flash by flexing rather than blowing the box to pieces...

Boc