With #13. His way is leave #13 on.

Place the ovals next to #13 and put them where you want it.

Mark the where the back of the ovals are on #13.

Remove exhaust, cut #13 at the mark. Put back exhaust.

Temporarily spot weld the tips to where you want them making sure its straight and where you wnat them.

Remove exhaust and finish welding.

Then buy those plastic rings that go around the tips to have a nice finished rear.