There are special nuts that probably came with the door uppers when they were purchased. The nuts are just held into the bottom of the door insert pins by spring pressure. Chances are, when the bolts were pushed down through the inserts, the nuts were knocked loose and may be in the bottom of your door. The nuts are just cylindrical on the outside and have three holes in the sides where the spring clips on the inserts kind of lock them into place. If you find the nuts, before you put them back in, run a 3/8 x 16 tap through them (should be the size of your bolts), clean the bolt threads with a wire brush and spray a little wd 40 or teflon spray on the threads before bolting it back together. When you insert the bolts, push in on them very gently and when there still is about 1/2" sticking up start turning the bolt.
I should have told you....pull the uppers off (straight up)(you may have to unscrew the bolts if any of them are still holding. Then you can pry the inserts out of the top of the door to see if the nuts are still on the bottom of them.
Hope you can find them, I've tried to find replacements, and can't find them. I was thinking about using all thread nuts and making my own.