Maybe I just can't see it, but you do not have to use the top slot, just use it where it is pinned in the picture.
I'm not sure that I would even want the mower moving forward or backward on its own anyway.

Think about what the mower actually does if-when it tilts backward and if you actually want the mower to move in that way.
I think that you think there is an issue when there isn't, just my opinion.
I'm pretty sure that there WILL be issues using the box blade if you go with a longer stroke top link.
Good luck with your decisions.