Help!

I've woven a tapestry(yeah) 29 x 37. I decided to finish the edges(bottom and top) by darning the warp threads back into the heading (it was already twined). Unfortunately the tapestry started to buckle along the horizontal(terrible:( I took out the darning and the darn thing was still buckled. I realized that in darning and undarning I pulled the warp threads so as to push the weft in tighter through out the piece. Now, I have tried to smooth this out with my hands running them along the warp threads trying to get the wefts to move back. However this has only helped a little bit. My next thought is to hang the piece for a day and let gravity help. Would ironing with steam and press cloth on the back help? I hate to really block this since I've never done that before and sounds pretty aggresive, would that screw it up further? Help please!