I just dealt with the same problem on my '64. I had cracks about 1/16" wide along the edge of the upper layer of metal. It's hard to weld these up because the metal that's left is very thin. I ended up cutting out the crack to about 1/8" wide and putting in a piece of wire from a coat hanger to fill it in, then mig welding over that. I was able to grind it down to look decent. Then I smoothed up with Bondo, including the bottom of the valley. Doesn't look perfect but good enough for a driver.

'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc