I wouldn't worry about that setup at all, provided the bed is securely mounted. Ideally, you want a frame extension to do it correctly.
For the ones that shun such an idea, you might want to look at how many RV and Toter builds are done.
There are some LONG frame extensions used, and frame failures are non-existent.
There are also hitch extensions that are used all the time,
that will put just as much or more leverage on the rear as a properly done extension.
Again, no frame failures to speak of.
At least with good C-channel frames on domestics.
There is good reason why EVERY photo posted in this thread showing a taco'd truck frame is an import.
Frame failure on domestic trucks is not a thing. Never has been.
Beat on anything enough, hard enough, and it will fail, but it is far from common.
Lots of benefits to simply extend the rear as well.
No frame splice needed, no custom driveshaft needed, extended fuel lines, brake lines, etc, not needed.
You also dont kill your turning radius, or your break-over angles.