Due to how the three pieces fit together, I found it necessary to install them on the vehicle and tack weld them in place, then remove for final welding.Use stainless steel bolts and they won't rust for next time, and use flange bolts to hold down the heat shield.
Mark the exhaust pipes with alignment marks before you cut off the flex pipe
The pipes are not a snug fit in the new flex pipe, so the angle as well the depth of insertion all make a major difference in the final fit.
Although stainless steel flange-head metric bolts do exist, I could not find them in stock at my local well-equipped hardware stores, the big-box hardware stores, or Fastenal... so I resorted to the more common zinc-plated steel flange-head bolts and applied an anti-seize paste. If I was willing to wait, I probably could have gotten some off eBay.