Wow that new sill looks great and the fatal rust you are worried about is about 1/10th the rust my Bronco had!
I had holes over my rear wheel arches that were so huge you could store loaves of french bread in them. It took an entire can of spray foam to fill them in!
I think that tub is in fine shape for a trip anywhere. I'd sit in the back seat without any fear at all.
I just talked to a friend of mine who has made your proposed trip once. The next time, he put his truck on the car ferry.