Sell or keep 03 6 mustang?
I personally would check the rear differential and have the fluid changed there because every time I had a whine whether accelerating, cruising, or slowing down for me it has always been the differential. I would also change the transmission fluid just to be on the safe side and to put to rest any doubts about it whining there. As for oil I would go with Royal Purple or Lucas Oil in the differential, some may disagree but once I changed the oil in my differential because of the whine and heard no difference but when I went with Royal Purple the sound went away within a few minutes.
Now after saying all that the problem could still lie within your transmission, I have not driven your car so you may be correct in assuming that the problem is located within your transmission or you may know that as fact. I still believe that you should change the fluids with the ones above and give that a try or if all else fails take it into a shop and have a mechanic who is able to witness the problem give you an answer/estimate and see if it is worth it to you. If you're going to give up on a newer Mustang with one problem then buddy you have another thing coming if you decide to go with an older Chevy vehicle, not to bash Chevy's or anything but if the vehicle was abused (which most sports cars are at one point or another) then you'll run into problem after problem.
But hey, if you do get your problem sorted out you could always put a supercharger on that V6 and hang with the elite, the thought of cruising down a street knowing that every car you pass has nothing on you! What a feeling!
Anyhow, hope this helps.