that add shows a 960 with the early powermaster gear decal, and it shows lp.
that tells us that at least by the end of the 00 run.. lp was available. i imagine if you walked into a ford dealer and put a bucket of cash on the desk at the end of the 00 run and said i wanted a lpg 660..e tc. they mighta built ya one. :)
but i don't know that for sure.
someone like tony jacobs might have more insight to that. so far we know the rowcrops at least could have it.