I have a 1972 chevelle, and I want to replace the trunk. How is that done?? Is there metal cutting involved???

I am planning the same thing on my 70 which is exactly as yours. I did some research and found that the 3 piece trunk pans give you the most metal. If yours is really bad you might want to consider the same. Mine came with the 4 braces as well.
I haven't cut anything out yet but I plan to only remove the bad metal leaving as many seams and as much original metal as possible.
I tried to locate a walkthrough of some kind but came up empty.
After you cut out the areas to replace you will need to cut your patches to match. The easiest way to fit the patch is to form an offset in the patch and mig weld the seam. The problem with this method is that you will see the extra material thickness from the bottom and it also creates a nice place for moisture to collect. The right way to put it in would be to cut the partch to match and butt weld the seams with a mig welder. WIth either option you will want to use small tacks skipping around the perimiter of the patch and possibly cooling the metal occasionally as you go. The less heat the less distortion. If you don't want to do the welding yourself, you could probably fit the panels and have someone do that for you.
If you come across some good information, by all means share it with me. The more I know upfront, the better I feel.

Dan Solomon
70 Chevelle

Been there done that. The best advice to make you very happy in the long run is to buy acomplete trunk kit. Three pans gas tank braces gas tank straps and the body mounts. Set those new pans in the trunk and use a black marker to out line the new parts to the old trunk. You may need a spot weld breaker for the wheel houses. Quarter to trunk filler Across the back of the trunk is your lock brace center and two end braces drill the spot welds out so you can get the truck ouy from under them. Its the best way.