With the old firewall cleared away, I scrounged up some material to patch the hole where the heater motor originally fit. The new Vintage Air setup came with a bolt-in patch, but I prefer to weld it in. Plus, the new contour for the passenger side of the firewall (see page 8 of this thread) will cross over the hole, so I want it smoothed out and ready to attach the new lip.
I have several choices for making a patch like this. Most of the time I just grab up a pair of tin snips, but for bigger jobs I'll occasionally use a Rodman nibbler attached to either a power drill or a cordless (when I'm away from the shop.) It doesn't leave a particularly smooth edge unless you're using a rip fence, but nothing works better for complex shapes. I also have a great little tool made by Sykes-Pickavant out of England. I really like it for long straight cuts, and it will also do curves if you first trim it close to the cut line. You can see that I used it to take the corners off the square piece and make a circle. Only problem is, you crave a cup of tea after using it.
