Square plenum in round opening and what IS this thing?

[seismicfish]

Ok, so I have finally decided upon an AC unit to put in the AS (Atwood 13.5). From what I can tell from the intallation manual, the apeture needs to be 14" square. Below is a shot of our central vent where I want to mount it. Outside dimensions of the vent are 14" but how does one get around the rounded corners? Do I have to cut the hole square?

On another note, about 16" to the aft of the vent, there is a little 1" diameter aluminum pipe sticking down through the ceiling (acutually roughly flush) with some odd little bob in it. What the *^(* is this?

[Bigventure]

I am going to guess. The factory AC option was a split system. That pipe was for the ac lines to the evaporator. The condenser was on the roof.

The new AC, just sets on the roof. There is no actual duct. You just need the square inches as required. Or your capacity will be reduced. There may be wiring hidden around the edge for the AC that comes out by the fuse box. Pull off the eucussion and you maybe find wires.