For the benefit of a larger audience, I'll say some things about a couple of installation options ...
First, the subfloor should be 5/8" plywood, or thicker plywood, if you're installing directly over 16" OC joists. (This is key especially if the joists are NOT 16" OC. You may need to either reinforce the subfloor with more joists or go with a thicker subfloor.) The plywood sheets should be laid with the grained outer plies at right angles to your joists. The djoining rows of plywood are to be staggered four feet (so your seems don't all line up together), and nailed every 6" along each joist with 7D, or larger nails. If you're installing directly over existing wood or strip floor, do sand the high spots and nail the old floor again to eliminate the squeaks or loose boards. Then, you can install your new planks at right angles to the old floor.
If you opt to install your NEW planks in the same direction as the existing flooring (which is your situation), install 1/2" plywood gapped 1/8" at the edges FIRST, and again, nail with 7D or larger nails every 6" at the edges and at 12" in BOTH directions and through the interior of each sheet.
For you, the key is to 'certify' the joists, their spacing, and their condition. Second, 'certify' the THICKNESS of the existing sub-floor AND planks. From there, you can now talk w/your contractor. My best to ya and hope this helps.