Most likely the floor joist do run front to back. Typically, this allows the furnace duct to be placed between the joists. If you have multiple floor ducts in a straight line, then that should confirm joist placement. You should also be able to confirm this from below, between the frame rail and the lower skirting. There is black plastic that protects the bottom of the floor assembly. Run your hand along this plastic, if you can't feel a joist every 16 inches running side-to-side, then they MUST run front-to-back.
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