I'm wondering if they'd be best if tack welded. If you use JB Weld and it gives, could it be hard to get the bolt out of the nut?
If you are referring to my earlier post, no, there is reason it would be more difficult to get the bolt in and out with JB Weld. Where I suggested JB Weld was NOT on a discrete nut but on a "nut bar". As its name suggests, a nut bar is typically a flat piece of of metal tapped in the appropriate places instead of a handful of loose nuts. If you JB Weld (or tack weld... or even tape) the nut bar to the back of the place it is needed, all the necessary tapped holes are in the correct place.
I have used homemade nut bars on many automotive applications where the factory installation called for multiple loose nuts. For example, on my Spitfire I made a simple 2-hole nut bar for the ignition coil. I used 3M double sided trim mounting tape to secure the nut bar to the underside of the dash bulkhead. After that, all I had to do was insert bolts from the top to secure the coil. With the stock arrangement this is a job easiest handled using two people. I'ts a one person job with the nut bar.
Also, if considering tack welding discrete nuts to the back of steel parts, remember to keep a tap handy. Hex nuts tend to distort during tack welding which can make it difficult for the bolt to fit the female threads.
The pictures below are of "factory made" nut bars. I have no idea what they happen to be for... they were just the first pictures I found.