I took it into the local RV shop here and had them do the final welds. We added a vertical plate installed along the open part of the structural frame to add rigidity at the axle mounting plate. This was recommended by Dexter.
I was inclined to close the small gap between the axle mounting plate and the outside edge of the frame, but the welder thought it would be best to leave it as is and fill the gap with a weld.
It seems that the axle has settled some and so now a gap is apparent between the axle and the axle mounting bracket on the interior side (see pic below - you can see a little bit of daylight coming through). Hopefully this isn't cause for concern as this is the final install.
We installed the axle by mounting it to the mounting brackets and tacking them on after measuring for alignment. Rather than continuing with the weld, it seems like the best thing would have been to drop the axle from the mounting plates and weld the brackets flush to the frame. It seems that the flex in the axle is taken up slowly.