Paul,  Have  you had a chance to think more about providing
access  to the pan bolts inside the S-beam?     I am at that point
in the  construction sequence where I would like to deal with this.

Jerry

Have you tried a 1/4 drive rachet with U joint and ball allen end?

That should work. Any welding on those flanges is a bad idea.
Better to put large holes in the side panels of the S-beam
to insert a box wrench or allen key. Also move the beam forward
to put the bolt holes closer to the center.

I don't have my Rhino computer with me.