LPG Spark Plugs

I've converted 10+ cars and have run LPG for probably 250,000 miles. Never used special spark plugs.

Currently running a 500+ bhp AMG on LPG as a daily. Standard plugs.

Going into a 1999 ford explorer 4.0 v6 just pricing things up at moment

Depends on the vehicle my old Vauxhall Combo with factory gas used to be gapped at 0.8mm rather than 1.0mm made a huge difference to the low rpm behaviour.

Does it run ok and idle smoothly on petrol? If not there's no way it's going to be ok on gas.
LPG needs a really good spark so old plugs, dodgy leads etc are a none starter
I would try the standard factory plugs unless they are the multiprong variety. If they are I would get a single prong one with the same heat range so you can adjust the gap.
If idle is lumpy or it splutters when asked to run on low throttle openings under load at standard settings them I would close the gaps slightly and see if that improves it.

It took me ages to figure it out for the combo and it was only because I stumbled across a listing online that showed a different gap for the LPG models I sorted it, that was coil on plug too so should have been a fantastic spark.

Likewise I've run stock plugs even on stock gaps but the cars did have platinum plugs.

I've once had a car where it would burn out the stock plugs every 5k without fail. The Iridium plugs helped here, but it's the only time I have come across the issue.

For what they cost, I'd try some new stock items first and see how you get on. As said, if your car isn't running right on petrol it won't run right on LPG.