Filling the seam down the bed.

[sams60gmc]

So I was at the local back to the '50s pre car show friday night and saw a pretty trick '64 swb. And I noticed it has the seam in the back of the bed filled just like mine, but then I noticed he has the long seam that runs all the way down the bed filled too. I want to do it with mine because it looked really smooth and tied in with the seam on the end. Just curious what he may have used or what to use. Bondo? Lead? Any help is apreciated guys.