I forgot about Victornet, they have left handed dies and taps in all the standard and metric sizes, fairly cheap too:
Inch size HS Left Hand Dies
That said, you must have mis-measured the bbl length. If the bbl was really only 15", you are currently in possession of an SBR and have just posted photographic proof on a public forum. Separating the upper and lower makes no difference, the upper is the serial numbered part and it was assembled as a rifle. I know you wouldn't have done that, so you must have mis-measured.