You need a minimum distance between rows equal to the height that you will allow. So for trees 8ft tall allow at least 8 ft between rows. This would allow about 4ft for the trees and 4ft to walk/work. This also accounts for shading. I'd space trees about 4-5ft apart in the row.

Wider rows might have an advantage if you have the room.

Fruitnut,

I'm glad that you haven't left this site. Thanks for the comment. I had plans on having each tree spaced 4ft on center from the other, 4 in each route. The only thing I was wondering was the spacing between. So if I have plans on keeping the trees sized to 6.5-7ft tall you're telling me 7ft between the rows. Now are you saying from trunk to trunk, or from end branch to end branch?

I've got peaches outdoors spaced 7ft trunk to trunk. But these are on a trellis so I'll be able to keep a narrow canopy. In your situation I'd feel better telling you 8ft trunk to trunk. With most things that's going to require regular summer pruning. With vigorous trees on good soil it could be difficult to keep the canopy open enough for disease management and quality fruit.