Following up on his subject as I am about to do the smoothing now.
I guess the best way to do it is to cut out the old part and put in a new one like the DSE one. Or do you just put the new sheetmetal on top of the old one and tack it in place.
Here's what I did with the firewall. I probably cut out way too much but I wanted to get rid of all the seams. I also neatly cut out the wiper motor mounting surface and added a patch. I'm relocating it under the fender.