Locating the welds can be a challenge. I used a junk putty knife to force the panels apart slightly, which will distort the metal at the spot welds.
Which cutter you use depends on the location you're working on. The left and right sides, you can use a spot weld cutter, a drill, or any method that preserves the outer sheetmetal. Along the trunk floor, you want to make sure you use something that'll leave the trunk floor/rear crossmember intact and not put holes in it. The spring loaded spot weld cutter will leave a disc of metal that needs to be removed, the other cutter you ordered will remove the metal as it cuts, but be sure to not go into the underlying panels too much.
Another great way to do this is to use an air file with a 1/2" belt. Makes short work of the spot welds. They're heavily used in professional auto body shops, and they're great for dressing down your rosette welds when you're done.
https://www.amazon.com/gp/product/B00L50MKJS
If you have a later D3ZZ NOS panel, it likely has the holes for the '73-only bumper filler panel, which need to be filled for a 71/72 car.
Lastly, and I can't stress this enough, fit, fit and refit *everything* before you commit to welding for keeps. Taillight panel, rear valance, taillights, trunk lid and end caps. Fit it all together to ensure you won't discover an issue after it's welded solid.