I row a cataraft with a 66 in. NRS frame. Because the frame is narrow, I've had to have custom sized dry boxes. I have been very pleased with my boxes make by Partner Steel in Pocatello. They have been very easy to work with, and I've gotten my boxes quickly. If you know Partner Steel stoves, then you know that they make quality river gear.
I can't speak to your floor issue, but I sized my boxes to leave about a 1/2 - 1 inch gap on the sides, between the box and tubes. I use the welded brackets because I didn't want to deal with the adjustable hanging supports. If you are just starting out, you may want to go the adjustable route until you are sure you have your set-up dialed in. The one thing about NRS frames that can be both a positive and a negative is their flexibility of set-up. They give you options to move things around, which can be good,but sometimes that leads to headaches & confusion. It might be worth setting up your boat in the driveway, throwing on the frame, and doing some measuring & tweaking. Use the specs on the boxes & coolers you are thinking about, move your bars around on the frame, then look at it to see if it makes sense. It's the old adage ......measure twice, cut once. Dry boxes & coolers cost $$$. You might want to PM Alex (kazak4X4), he has a really sweet raft set-up and could maybe send you some pictures & ideas.