69-72 Hood dip

[StingRay]

I have a 69-72 GM truck hood that has a dip in the middle. This is pretty common but I've never seen anybody actually address a repair of this condition. On the pass side of the rib that runs front to back it actually has a dip. On the drivers it's lost it's crown but isn't concave. The rib itself is fairly flat in the middle section where I'd expect it to have a small crown. I have a few theories but am frankly scared to test them out. It's a very low crown panel and one wrong move it will be scrap.

There are the supports that run front to back either side of the center rib. They typically have foam between them and the skin. I've tried a few different shims to see what difference is made. Shims stiffen the hood but do not change the condition of the hood.

I'm thinking that over time the hood fluttering up and down in the middle at the rib (and they do flutter) could have any or all of the following conditions. Stretched material at the bottom of the rib as though a hat channel had it's lower flanges stretched. The rib could have over time sagged and pushed the front and rear of the hood out like a sagged roof truss. This would create a false stretch situation. The rib could have flattened out sideways pushing material on the hood side to side a little also creating a false stretch. While possible I don't see the area that is concave as actually being stretched itself and am leaning towards it being a false stretch because of another condition.

It's a pretty nice hood as these go and I'd like to make it perfect. The funny color of the hood in the pics is because it has been metal prepped and there is zinc phosphate on the surface.

I'm not looking to add another brace. Has anybody actually corrected the problem that typically causes this?

[StingRay]

So here's how I tackled this.

I cut a piece of 3/4 Fir plywood with an arc of about 1/8" over 33" I set the hood on it at the rib. I used some different diameters of pipe up against the radius side of the rib with a dead blow hammer to work the rib a bit. This pulled the material in a bit and put a bit of crown back in the rib. I increased the radius of the wooden rib a couple times as I worked through it. This got the surrounding metal back to no dip. Then while still on the wooden rib I used a light slapper from the bottom on the area next to the rib to help rearrange the metal a bit. This brought the lowest area on the pass side up a little more. I washed over the area afterwards very lightly with a shrinking disc. I just got it hot enough to steam a little bit. Didn't even get color. I had a number of small dings to clean up and a couple were inaccessible from the back side as the there is extensive inner structure at the front and back. I used the Unispotter to pull those. I use very short burst on the trigger so the pins will come off clean and minimize any hardening. . I'll get some primer on it and glue the new foam back in under the supports. I'm pretty happy with the results but this noodle is going to be ugly to block sand. I'll need a sanding block with a hover feature.