according to my oldest Champ catalog (1981), HO-3 is the coldest 14mm 3/8 reach plug in the platinum/gold palladium list. Next up is UJ-7G, and next is HO-8/HO-8A.
According to that list, the nearest crosses for HO-3 in the std would be J2J and UJ2 (a bit cooler yet) UJ81J and J4, UJ4 (a hair warmer.
While this and some newer catalogs do not specifically list 990G, it does list several other models of a similar age reccomending J6, J6J for most of them. There is a note suggesting use of J4/J4J for cold fouling.
In the racing plug section there are J60G and J64G plugs listed at about the heat range you're looking for in the HO-3, slightly warmer than UJ7-G.
Trouble is I bet that most or all of these gold types are very hard to come by. Champion has been sold to Federal-Mogul (I believe) who has had some pretty severe troubles lately. To my knowledge they haven't even printed a current catalog like you see in the spark plug section of most stores in two years. I recently saw an article outlining the procedure for bushing Ford plug holes down to accept 14mm plugs as many cooler heat range (racing) Ford plugs are gone. The current Champion website suggests they are doing us all a favor in keeping costs down by consolidation and pruning the slow-movers out.
The good news, if there is any, is that few of us need those old monsters to earn a living with, just start 'em for giggles. If we fouled a plug now and then, we'd live with it. Probably not very likely we'd work one hard enough to hole a piston from too hot a plug.