I don't know much about metal detectors, but I do know something about using junk blades on junk wood.
Are you going to be ripping these with two cuts on a table saw, or are you going to do it with a bandsaw? It's probably cheaper if the former to just use a table saw blade that is inexpensive. If it finds a nail, you'll know it.
If these were beams from a very old building, they should be pinned mortise and tenon (all wood) and any nails that are in them should be incidental like a place to hang feed bags or twine or wire, or perhaps a fixture for a ladder, etc.