Growing your plants out from under the canopy of the walnut trees would reduce the competition for sunlight and the chance of allelopathic affects from the fallen walnut leaves.
Because the roots of the trees can extend much farther than the foliar canopy, you may encounter tree root competition for water and nutrients, plus possible allelopathy from the root exudates.
It may be safer to plant in raised beds. But as you may know, there are other possibilities for wilting of tomato plants. Water deficit would be one; inadequate irrigation, or tree roots competition