It is easy to find recommendations for spacing pecan trees in a new pecan planting. I usually recommend an initial spacing of 35 to 40 feet between trees when setting out a new orchard. But I've never seen anyone talk about the space that should be left between a new planting and established pecan trees or adjacent woodland.
In the photo above, a row of young Kanza trees (left and west side) are growing next to a block of large native pecan trees (right and east side). Note how the Kanza trees seem to be leaning away from the natives. Even though the limbs of Kanza and native trees are no where near touching, the large native trees seem to exert their presence in forcing the Kanza tree grow westward, away from the crowns of the larger trees. There are 45 to 50 feet between native and Kanza trees pictured above.
Now let's look at another situation. The photo above shows a row of grafted trees (left and west) adjacent to woodland (right and east). The distance between pecan trees and woodland is 60 to 65 feet. In this case, the pecan trees have had enough space to grow normally without obvious influence from the large trees in the adjacent forest.
If you are establishing a new pecan planting, my recommendation would be to plant no closer than 60 feet from adjacent established trees (native pecans or woodland). This spacing should ensure balanced top growth of your new trees.