Post some pictures of your dents, so we can give you more than rules of thumb. So, some dents can be pulled out using automotive "paintless dent removal" techniques. I have seen examples where it is very difficult to see where the dent used to be. Most dents won't simply "pop" out, and if they do, they tend to pop back in again. Any dent will result in stretching the aluminum, if only a bit. If the dent results in a crease, you will likely not be able to work the metal back into shape without at least removing the interior wall and insulation, and using some type of roller to work out the dent from the backside. It doesn't take a very dramatic dent to result in the need to replace the segment or panel, and this is typically the way to restore the appearance of the trailer to "like new." In the case of your '69, you can still buy the 2024 T3 Alclad aluminum that the flat panels are made of, but it is very unlikely that you will be able to buy a "new" formed segment (that makes up the rounded ends of the trailer) that will both fit and be made of an alloy like yours. So if it comes to a segment replacement, you have three choices: work the dent out as best you can, and tolerate some imperfection, find a used part made of the same alloy and use it as a replacement, or remove the segment, and find someone with and "English Wheel" who has the expertise to work the dent out without changing the overall shape of the segment.