I'd suggest you hold the extension around the bumper and mark on the bumper where you need to cut the slots. That's what i am planning on doing

 

So I got the "new" bumper and took my old one apart and then I got my measuring tape and measured twice and marked once. Just to double check, I'm going to do the same before I cut it. The hardest part was the riveted metal pieces that hold the extension on the back were a PITA.

 

How do those metal pieces go on?

 

sorry, I have not be on here in a while. Those metal pieces are rivited on with metal rivits. I do not know the size of them yet since I havent had a chance to get to that far in the rebuild process and I have since started on the easier parts of the metal work that needs to get done and will take a lot longer to complete.