I think you have perhaps revealed too much already. Learn all you can about 2 box detectors in the time you have, and don't take anyone besides your assistant along. Try to think like the one who buried it, where would you hide this burden, this chore you must do before you die? You seem to have the area fairly narrowed down, go there, look around, imagine you have 2 nephews/ sons, whatever, that you don't trust yet, and need to hide this thing from them and others. How far could you lug a box of gold while they are off doing ?, what were they doing, for how long were they out of sight?
Further, how much money can you spend to find it, a big factor. Rhetorical, you need to answer that one yourself. What is it worth to you, to put it another way. From all you've said so far, it seems to be a very real thing, with living eye- witnesses. If you don't doubt the story, why not look for it off and on when you can, even if it takes years? Why only this one time? If this story were in my family, and I believed any of it, I would be living there and searching every spare minute I had. It can't be that hard to find. Keep the search to yourself if you don't plan to share it with the rest of the family. By the way, that is part of the plan also, what will you do with it if you find it? What has prevented others in the family from doing what you propose to do? In other words, why haven't any of them bought a detector and searched for it? Or hired someone to search for it? I like the idea of hiring the oil and gas surveyors, and having them show you on paper what's underground, their focus would be on something else.
Will you be safe there during the time you will be searching? Can you protect yourself? If not, I would consider hiring some security. How will you transport it? Who will you take it to to get the best price for it? I suggest one of the large coin auction houses, such as Bowers & Merena, or an auction house, Sotheby's, Christies, or Butterfield's out west there. If you believe in this, plan your work, then work your plan, just be a lot more discreet about it. I certainly wouldn't invite any strangers to come help me find such a treasure, you are begging for trouble if you do.