I recommend the 6" base. Lay it in 2 layers. Put the first layer down, tamp it (use a power tamper for large areas). Then second layer of gravel, and tamp it. Our front walk is made of very large stone similar to yours, and is 15 years old. It was done on a skimpy 2" bed of gravel. It mostly looks good, but 15 years of frost heave have made the stone shift a bit. Not too much because the stones are so large. The back patio had 8" of gravel and is 4 years old, but looks brand new, with no shifting at all.

I started putting down a patio and sidewalk too out of sandstone last summer with the largest piece being about 3 feet wide and about 4 inches thick and we've only put down about 2 inches of sand to bed them in. We put in some pea gravel in the joints but after looking at it we come to the conclusion that we don't like the look of it. The pea gravel has too many colors in it and your eye immediately goes to the gravel and not the sandstone. So I think we're going to just have the natural look of moss with being in a woods here in PA. We didn't get all of it finished last summer so we've been working on it the last couple weeks and have made a good dent in it. I've done some research and I thought just bedding them in some sand would be fine. They didn't seem to shift or move any this winter. We edged it with some of the plastic edging just to keep the sand in place and then will plant grass up to it so we're hoping you won't see much or any of the edging. I'm new at this and thought that bedding it in gravel that the sandstone would shift or move a little.