After quite a bit of fitting tonight, I have these two brackets partially fitted between adjacent ribs.

By the way, here is one of four pair of auxiliary seat belt brackets that I had made up. I got three quotes and went with the low bidder. The cost was about $80 for all four pair. Actually they made two extra pairs for free. Unless I screw a set of these up, they're available to anyone that wants them for the cost of shipping.

I clecoed on the floor pan to check the fit of the first bracket.

Not bad...there's a little gap at the top, but that should pull down once I put the pop rivets in.