1 hr. riveting the floor ribs to the gear mounting channel (GMC) (backside) and its uprights.

The floor ribs are riveted to the GMC from the outside.  The rivets on the top will be riveted from inside the GMC.

The instructions say to rivet these from outside the GMC, but the bottom 4 are countersunk and are riveted from the inside.  The next one up is a domed rivet and I've seen photos of it being riveted from the inside, so I did the same.  The rest are riveted from the outside.

Investing why the 2 tubes here are tight, so I removed the outside bushing and we can see that the slots are clearly not coaxial.

Removed the tubes and using a steel rod we can see the problem.  Note how the two closer ribs are touching the rod, but the middle two are at least a 1/4 inch away.  This happened because if you recall I had to do some serious flange bending to get the channel to fit under those two middle ribs so it brought the slots closer to the channel.  So, working from the center out I will start riveting the ribs to the GMC and then get a more accurate measurement of how off the two center ribs are.  Then I will have to grind down the left sides of slots (bottoms seen here and the closing ribs which are not in the picture).