There was no reason to use fine treads on a truck. But I know they are used. A little extra strength was not needed. Possibly on a heavy door but the numbers don't support that. No lack of thickness for threads. No fine adjustments needed. Captive nuts usually have some available movement. Captive nuts are used for that reason sometimes. I would hope the vendor used a parts manual/list for sizing the kit.
The proper way to recover from a bad kit is to go to a hardware store and buy the quantities needed with no shipping. Glad you had a stash to draw from.
Personally I would never, and have never, bought a kit. I save the hardware I take off, I also have a stash and I go to the store occasionally.. It's pretty easy to figure size, pitch, length. (thread gage, another bolt or nut to try and calipers).
At that time in history, quality control for civilian trucks was not a priority. Hardware was probably scarce. GM got what they could get and used it. May not match the released parts list. Would be interesting to check.
