I know you can paint over some seam sealers,but assuming you're wanting the best finish possible,welding them up would be the best option.
You'll need to watch out for warping though.

Alternatively,try and find someone who can do lead work.
Lead allows for vibration and flexing,better than most bondo fillers.
Or...if you're feeling brave,have a go yourself!

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.