Building on the idea from the last post, I suggest the following:
1. I presume the huge dents you're referring to are the areas where it bulges out between the mid reinforcement and the end caps.
2. I presume that the corrugations are relatively unmolested very near the end caps.
3. If you can remove the end caps ( I think you already have??) you need to trace inner profile onto a piece of wood that you can cut to form a sheet metal "buck" or "hammerform". This would preferably be hardwood, and can be made of multiple layers to create a buck several inches thick.
4. You'll also want to create a "die" stamp by tracing a portion of the outer profile. The die only needs to be two inner corrugations and the outer corrugation between them. will look something like \/\/.
5. Secure the hammer form in a large vice, or anchor it to a good solid workbench surface. Place the caisson over the hammerform, taking care to center it over the form, and with the corrugations aligned with those of the hammerform.
6. Working back and forth along the center line of the caisson, use the "die" you created, and a dead blow hammer to slowly and carefully restore the corrugations. Continue the process, slowly working out from the centerline,and toward the out edges. It is critical that you take your time, and ONLY hammer over the buck. having it underneath will help to insure that you don't end up dishing the caisson in.
7. It would also be helpful to use a steel straight edge along the length of the caisson, to help you gauge how bowed or straight it is.
Probably clear as mud... if you need it, I can put together a sketch of the busk and die I described above.
Pvt. Kirk Gustafson
23rd Regt. 2nd Infantry Div. WWII HRS
1942 Chevy G506 1-1/2 ton 4x4 Cargo w/ winch
194? Ben Hur water trailer