Fixing bowed tailgate?

[Number21]

The tailgate on my '79 is in pretty good condition, except that it's bowed out a little bit in the center like somebody put something too heavy on it. Anybody have any thoughts on how to fix that? It's not very bad, you have to be looking for it to notice.

I was thinking maybe trying to hook a heavy strap between the tailgate and the front of the bed, and ratcheting it as tight as I can. I'll probably just end up bending the other end of the bed...