What I did here is put a little ruler to show the width of where I wanted to drill the holes in the pan. I taped a 3/8" drill bit up to it. The flash obliterated the marks on the ruler. What I wanted to do is get a visual of the 3/8" hole and how close to the edge of the support and the reason for all this monkey diddlin' around was I wanted to emphasize how important this hole is in relation to the outside of the support. The reason it's important is you want to get that plug weld as close to the edge of the support as you can then when you're done welding you won't have any weld on the very edge of it, and if you take a close look at the bottom of the floor pans you'll never know it was welded.
When I had the pan in to show you that little ruler, I marked the edge of the supports on the bottom of the pan with a magic marker. I think I used about 10 screws total to hold them down to mark them. I also put a scribe line through the magic marker line. You see the line at the 5" mark, take a magic marker and put another line beside it 3/8" from your support line, you can barely see the little mark at 5-3/8" on the ruler.
I took the pan out and took 1/8" drill bit and drilled a hole in the pan 3/8 of an inch from the edge of the support, then I chase drilled it with a 3/8" drill bit. I did about 5 holes, put the pan back in, put a screw on each side of the hole and marked it with a magic marker, tight to the hole. What you're looking at here is 1/8" between the edge of the support and the edge of that hole in the pan. It took a lot of explaining here to do a simple task of measuring that distance but after these are welded in they will be tight on the pan and you won't see where they've been welded. The bottom line here is I just wanted to show you how I came up with that distance.