A roof skin on this truck, in its original form, has a crown in both the x and y axis. This crown is what provides the structural support of the roof skin. Kevin's pictures show that the roof has been neglected or to be more accurate, used as a ladder or something. Any dent in the roof is going to interrupt the crown where the skin will lose the support it once had.
Any dent in that roof skin, or any panel for that matter, will typically cause a stretch as it gets pushed inward. About 99% of roof dents are caused from the outside and push inward. When removing said dents, some shrinking needs to occur in conjunction with bumping the dent. Looking at the bare spots (dark circles) in image 5140 on your flicker page, these suggest that dents were hammered upward, likely into a hard dolly?, with little if any shrinking occurring. The fact that the paint is sanded off reinforces that these are stretched and when they were hammered upward, may have been stretched even more. Looking at the image 5140 again, the center of the panel is shown to be recessed in comparison to the flat strip of metal.
Please understand that oil cans are a chore in/of themselves when the panel is in front of you, much less looking at pictures. But what I perceive has occurred is that the extensive dents and stretching that went with them, combined with hammering the dents upward (possible more stretching), has a bit of excess metal looking for somewhere to go. This is normally seen in the form of a sinusoidal wave, and as there was less damage in the middle to lock things in place as occurred with the dents, the center was pushed downward. My thoughts are the large center oil can (low) is a side effect of all the stretching from dent formation and dent removal with not sufficient shrinking to counter those effects. I would start by leaving the side effect (large low oil can) in the center alone, and attempt to shrink each high spot to relieve some of the pressure pushing outward. Use donut dolly or shrinking tip on a dent puller, I'd shy away from a torch as I don't think you want excessive shrinking going on which may send you to the other end of the spectrum. Slow and steady shrinking to sneak up on the repair. Enough shrinking to either side of the center of the obvious and larger of the dents should show progress in the form of the oil can starts to loosen up slightly. Once some of this pressure is relieved, then start some off dolly bumping where you may see some highs and lows adjacent to each other. The other thing to keep in mind is that any localized low adjacent to high spots (bare) are themselves high spots when you look from the bottom. So they will likely need shrinking as well.
