Ok, based on those pics, he's not 50". Maybe 50 FISH inches. ;) :P
If you can, take a pic of him stretched out in his new tub, since it is a known length. You could also let him crawl along a wall (helps if it's in between the wall & something - i.e. an empty tote), which will often encourage a snake to stretch out straight. Just mark off head & tail against the wall & measure in between. :)