I would say size 19 was your best bet. Basing on road bike size is a mistake, unless you intend to ride your mtb as if you wished it was a road bike. The most dynamic mtb posture is more upright and therefore needs a shorter saddle to bar length, whereas the length you need with a road bike has to be suitable for stretched out head-down charging.
I would always suggest the smallest size that Kona would recommend, which in your case is 19. Never use a stem just to achieve perfect fit - a short stem will give much livelier steering, which you may find unduly twitchy coming from a road bike. I personally ride several sizes of Kona, but my favourite is one that according to Kona is too small for me. It's perfectly comfortable, and there is no doubt at all that the smaller the frame the more fun a bike is to ride - the length of wheelbase is really important to the fun factor.
The other thing to bear in mind if you fancy a 90-92 Kona is that they sized them differently then and most owners (unlike Hilts) don't know what size they are selling - i.e., if you're offered a 1992 size 20 Kona, the odds are it's probably a size 19 as Kona measured at that time.