I've fixed dents by putting air pressure in the pipe and heating with a torch. I believe I was in the 60psi range to get them to come out. It discolored the pipe. I made caps out of .060 sheet metal and put a valve stem in one end, used a muffler clamp that fits the big end of the pipe and also put a piece of old inner tube under the cap. Did basically the same for the small end but had to use water pump type clamps (band clamps) and made the cap long enough to get two clamps on it. I don't know if this is the safest way but it worked. Also stayed clear of the ends when working on the dents.
