I would have thought someone here would have responded to you by today/tonight. There has to be someone who has installed the Carr hoop steps??
I have not looked at the GX2's like you are installing, but the install alignment to enter the cab should be just about/or the same as the open hoops.
I think you want to line them up with stepping into the open floor areas, or at least the foreward most side lined up with the foreward most floor opening, it would make entry real smooth - step up and into where they line up together.
I was thinking about installing the hoops, except they do not list the Mega Cab, only the Standard Cab and the Quad Cab. I e-mailled Carr asking if they would install on the Mega Cab, was there something different about the bottom/floor area, etc that made the Mega Cab different than the other two cabs? Possibly that they needed to update their site for the Mega Cabs? Long story short, they have not responded to my e-mail questions, must not care if us Mega people buy their stuff or not??? Their lack of response kind of suggests they don't want the Mega people as customers. Truly a shame, lots of us Mega owners out there, lost sales I guess.
Maybe by bringing your post up to the top it will get some attention and answer to your question. The Carr site does have mounting/installations instructions based on the part number of the steps you have, maybe their information has some dimensions for you to use? Or at least a pic of them installed for you to use as a reference?
CD