I don’t recall it completely blocking access
I put my box together and put the tailgate on last without removing anything
But it’s been a while I would need to get photos next time I’m near my trucks
But I think I know what you mean
Maybe something is just bent? Or my hands are small

I’ll see if I have photos in my computer
Update
So I don’t have a good photo, but what I see is that I vaguely recall three bolts to hold the box edge to the cross brace (bottom back of box)
And that includes only the lower hinge bolt
So... maybe you (and I) did have to install the top bolt of the hinge before the back cross brace goes in and I see in my partial box compete that I had both hinges on with no tailgate
But then how did I get that tailgate on?
I didn’t take the cross brace off
I’m wondering if I had captive nuts for the top bolt on the hinge which was likely original based on other folks comments
Memory is failing me now
Maybe I managed to sneak a wrench up in there blind just to loosen that top bolt but not undo it completely when I installed the tail gate
-s