Inner rocker panel

[biohazardbill]

Hi,
My first attempt at replacing body panels. I ordered outer rocker panel, cab corner, lower front pillar and the ineer rocker panel. The only piece I am having issues with is the inner rocker. I can see where it goes but my old one was so far gone that I cannot tell what if anything it was attached to. Is the "inner rocker" part of the outer cab floor? If so would it be easier to replace that piece instead?

This might belong in body work, but it is a specific question to this year truck...

Thanks for any help.