i have had this problem for abit, my car some times back-fires real bad, (tore the old air box in half), i was told its the spark plugs, and that theres a specific brand, anyone know?

Check your leads too. Use NGK plugs, part number ZGR6B-11 and regap them to 0.9mm

LPG backfires are usually ignition related.
Plugs, leads.
When my dizzy cap was on the way out, it showed up when on gas it would backfire pretty bad.

Alot of gas installers put a 'pressure relief' in the airbox, to prevent the box being torn apart from an intake backfire.
I've seen it be something as simple/dodgy as a hole in the airbox covered with a rubber flap that's attached by rivets on one side, to mine, which has a spring loaded valve that pops open when the airbox has positive pressure.
A little mod like this might be something to think about, if only to prevent constant trips to the wreckers for new airboxes!

new plugs part number in previous post, xr6 spec.
new leads, ngk, bosch, eagle etc, just make sure they are 8mm spiral core.

and if it has not been done in a while get the gas tuned.

as steady said put something on to release the preassure in the intake for a backfire,
a few holes in piping with wetsuit material sleeve over it is pretty common.
you can get spring clips from ford which hold the airbox lid down and let it lift.

Popped a hood on an LPG 4wd once - guy used a stubby holder over the airbox pipe as his backfire arrestor

yup must agree with everyone... plugs, leads, check cond. of dizzy cap, and just put a flapper valve in the air box, there aint nothing dodgy about that, prolly one of the most effective systems i've seen...

Ford sell a spring type clip to replace the original push over type ones that hold the lid down on the air box but still alow it to lift slightly if a back fire occurs, I have them on mine and have had no problems as yet.

didn't stop mine from blowing apart, made a flap relief valve for mine, bought another section of ducting from ebay for $30 instead of $100 from the wreckers or $200 from ford