I had my wife go to Harbor Freight while I was working yesterday and pick up a tap and die set since I didn't have any metric taps at all. I got a free tape measure too!
I was lacking thread cutting oil. I read on Practical Machinist that bacon grease is a good substitute for thread cutting oil (way better than WD-40, PB Blaster, bearing grease, etc.). Fortunately, we keep a container of bacon grease in the fridge for cast iron cooking, so I scooped a spoonful onto a paper towel and took it to the garage for tapping a few holes.
I used the M6 tap to tap the gas tank mounting stub and then I welded it onto the frame perpendicular and square like the original.
I also cut up a piece of metal and made two tabs that I could weld to the frame for the skid plate to mount to rather than using the aluminum clamps around the frame that destroyed the paint on it over time. They look uneven because they are. The holes in the skid plate are offset for some reason. The M8 tap worked very well for tapping the holes in the tabs. The bolts I'm using to show it assembled are not the bolts that I'll be using on final assembly.
Now, off to the powder coating place.