Captured nuts under a bed

Now that I have your attention - There are a couple of 5/16" pre-existing holes in the bed of my pickup near the tailgate that I'd like to use to mount a QR bike rack setup. To really make this work I need to have captured nuts under the truck that I can put a bolt in from the top without crawling under the truck to do it. The goal is to be able to quickly pull the rack and leave no protrusions in the bed for cargo to catch on.What's the best way to secure the nuts under the bed? My first thought was to tack weld them in, but I'm afraid that would encourage rust of the bed and I'd prefer to use stainless nuts for longevity. They would end up being outside the frame rails near the rear wheels, so road spray and dirt are likely.

Tack and paint?  JB weld them in? Some other wizardry? What do you guys think?

Rivnuts?

Is there space for a separate bracket that would clamp to the frame and support the captured nut?

I'm just think that if you go with a separate bracket, you can replace it if it rusts ( without damaging the bed), and also have more option for tack welding, crimping, or pem nuts. 

Rivnuts - that may be the right answer although 1/4" stainless are gonna need a serious HD rivet gun. I wonder if our shop at work has one.....

You know, I could probably get away with the zinc coated rivnuts, or even aluminum. If they get buggered it would be easier to drill them out.

IF you don't want to mess with huge rivets, U-Nuts or Clip Nuts are available at most auto parts stores or hardware stores, and they can be removed/replaced easily.

real hard to put that in the middle of a truck bed floor tough.

There is an alternative to traditional riv-nuts that can be installed/removed easier. These pull a threaded sleeve into a collar which deforms to capture it, They have a thin top edge that can be drilled out quickly/easily. They do not require a lot of effort to install.

Stainless riv-nut equivalent

No harder than any other flat piece of metal. 2 holes/clip. 1 to slide it through, and a second for the bolt.

Weld some nuts to a backing plate, then screw the backing plate to the bed.