If the row run north--south, and you have no other trees that shades them they will do fine, be sure to feed and water them properly.

Farms that grow these type of walnuts (zone 6) plant them as close as 28' and get great production. I planted 3 Northern Pecan (Carya illinoensis) this year and used time release (2-year) fertalizer tabs designed for nut/fruit trees and they have grown 2'.

The following link explains how to properly care/prune nut trees--about 1/2 way down the page.

Here is a link that might be useful: How to groww...

slinger,
You're probably OK.
About 9 years ago, I transplanted several hundred northern pecan & black walnut seedlings into a CRP riparian bufferstrip area along the eastern edge of our farm - about 7 acres total. I put them at 20 ft spacings, in two rows 30 ft apart. Had intended to come back and topwork every-other-one to named varieties and remove less-desirable seedlings when canopies began to crowd. I've not gotten around to grafting all of them, and some of the BW seedlings are now large enough to bear nuts. Even at 20 ft spacings, it's still going to be several years before canopies touch, and with open space on one or both sides, I may give removal of ungrafted seedlings second thought.