I put a couple of 2x4's under the pan with a couple of soft tipped wood clamps, it makes it easier when you drill the holes.
You can see some 3/8" holes for the plug welds here, and if you look close, you can see where I took the tin snips and cut a little "v" on the very end. The little "v" will give you something to weld to. I like to keep them tight right on the end.
I put a couple of 2x4's and a 4x4 under this. It works better when you're drilling the holes.
You will have better luck if you use 3 drill bits. I use a spring-loaded punch because I don't want to use a hammer on these pans because they're molded so nice. The little tiny drill bit will start out better with a small punch mark.
Here is a spring loaded punch marker.
This will give you a good idea of how I spaced my plug weld holes.