Rough door opening alongside cold air return

I have a question about framing an interior French door in a load bearing wall. The door is 60" wide to be centered in the middle of a 12' wide wall that was once an external wall. An addition was added later, and the door will be installed to open up the wall between the addition and the dining room. There is a second floor above the dining room, but the addition is a one floor 12'x17' room.
First, I am planning on using a 2x8 header above the door. Is this the correct size stock to use for a 60" opening in a bearing wall. Also, do I need more than one jack stud on each end of the header, or is one jack stud enough to carry the load.
Next, I know that, ideally, the jack and king studs would sit on the sole plate that would extend from the outside of the rough opening all the way to the wall. In my case, though, this is not the case. The rough opening of my door will be directly next to a series of cold air returns that run to the upstairs and would be extremely difficult to move. This will put the king and jack studs floating directly over empty space where the sole plate and subfloor were cut out to accommodate the cold air returns. How should I go about supporting the frame? I can really only use the floor joists underneath the door to support the load, because, as I said, I cannot install any framing outside of the rough opening.
I was considering just running a plate underneath the door and setting both sides of the framing onto this plate. I would think that this would be a better scenario than dropping the studs all the way to the foundation because it will distribute the load over a greater area of the foundation through the floor joists. Although, if I go with this method, I will probably have to shave 3/4" off the floor joists so the plate will sit at the same height as the sub floor. That way, I wouldn't be creating a trip hazard.
Any help on this subject will be greatly appreciated.
Justin Miller.
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you have described some things that sound very dangerous, structurally. I would get a pro involved in this. bill

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