I own most of the tools and would hate to buy a new one for this project but take your best shot at the best way and tool to do this and I will go from there.
I have not actually measured it but I think the main trunk line I need to cut is 12x18, might be 12x24. Standard gauge metal HVAC duct from 40+ years ago. I have reasonable access on all four sides. Well that one side is going to be a b**** but I can do it once if I must. Most likely, I want to return the duct to its original position after the project is completed.. I might choose to step it up to make it easier on the next fat guy. But that part I can figure out.
I can't unzip it at the seams without a lot of digging in 60+ year dried dirt as the next seam is resting on the ground. Hey, I did not do the original install. If I had it would be far different.
So what I want is a clean way to cut the duct without destroying the metal on either side of the cut. As an example: much like cutting the four sides of a wood box with a table saw.
Now to inject a little humor to the thread; the reason I want to do this is because my belly has grown to big and my back to bad to scoot under the tunnel I made years ago in this crawl space. Did I mention I hate tight crawl spaces.
Suggest a diet and die. There isn't time or the willpower to make it happen. I got to get over there to figure out where the water leak is coming from. Go ahead and make your fat guy jokes if you must. Paybacks can be awesome. -:)