Bags are not your fix. You will be tearing up a good old truck. Air components are expensive and can tear/break and can't possibly be as long lasting as steel springs, etc. You didn't buy an old truck to make it ride like a Benz. First figure out if it's body mounts rusted or rubber disintegrated as Cletis says. Find out if it's saggy springs or a broken bracket/shackles as tom says. That's just to get it level again. Of course there are many worn parts on a 45 truck. Fix as needed to do the type of driving you do with it. Check that all tires are the same diameter. Shocks don't cause sag. I don't know much about air bag installation either but I am guessing a coil spring vehicle is what they are designed for not a leaf spring vehicle. They don't make a bag "kit" for your truck so you will have to "build" a system and make things. If you are going rod truck/show truck then maybe you want bags. To fix a sag, get salvage parts or shop for good price on new. AFTER you are sure what's wrong.
Just buy what you need, shackle bushings or a leaf spring for one side/one corner or body mount kit. Why would you need to buy new U bolts? You may find a whole donor chassis. Trucks that old have rusted out bodies beyond repair but the chassis may be cheap or free. Not many 45's (build in 44) were built except for the military. Some were let out to farms and such. I guess 39-46 are same spring parts? The parts wanted forum here could be very helpful.
The good news is some of the spring shackles are 34-55 and the spring bolt and bushing is 41-55. Which could mean many source options. NAPA, O'Reilly's. Rockauto, speedway, Amazon, etc.

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