MrFixit is right, but just to be a bit more descriptive: The corner vents and the center vent are fed from the same plastic ductwork.. You could trim the duct at the center exit and block it off with tin or something, then attach two round exits to the duct on or near the block-off plate as Roy's done with the PVC. Or even eliminate them completely by leaving them blocked off.
As for relocating the HVAC, the wiring is the easy part to extend. The heat selector cable is pretty limited in length, I doubt you could move it more than a foor from the stock location. The good news is that the ends are pretty much common to most GM vehicles of the era, if you could find something with a longer cable (I would check something like the Pontiac Transport) you could easily get more room to move the controls. Be very careful not to bend this cable unnaturally - they get tough to move if you kink the inner wire and you could wind up snapping the plastic lever on your control. Trust me on that one. 
HTH. And some pics would be cool to see.
