That material is made out of random wood, pulped and held together with natural lignin. How can this possibly be archival? It is also sensitive to both temperature and humidity, constantly changing size and form - most likely at a different rate to your print.
I'd suggest finding some 10mm foamboard or laminating up a couple of pieces of 5mm (which would also allow arranging the joins from smaller, and cheaper, sheets in a non-overlapping way). Foamboard is more rigid, more archival and lighter in weight. This structure can then be laid down flat (on a perfectly clean and flat surface) and the backing frame you mentioned stuck on the back, ideally with an extra piece of framing crossways, halfway along the long direction, for some extra centre support.
Making the bond between the foamboard and the frame can be done with a suitable white-glue. Putting the print on to the foamboard is another question. I have used 3M Photo Mount with large prints (a panorama of five 20x16, trimmed down somewhat for joining up), but am wondering how long it will last before peeling (three and a half years so far). A better solution for a single print would be to go to a mounting shop and have the print stuck down with one of the double-sided adhesive films using the appropriate laminating/mounting system.
Edit: I forgot to mention, foamboard is also available with one self-adhesive surface, but whether that is practical depends on what you have available locally. A mounting roller will make this material (and the other options) much less fraught.
Last edited by MartinP; 10-08-2013 at 02:40 AM. Click to view previous post history.