Sorry but don't remember the rivet sizes. Measure the holes accurately, get the head type from other rivets on the frame. The minimum length needed is the length of the hole plus the diameter of the rivet. You can cut to the proper length if too long but can't add easily. Then you will need the proper tools to buck the head and form the end. I typically use an air chisel with rivet tool. Most are installed cold.
I have obtained rivets from Hansen Rivet, McMaster-Carr and Big Flats Rivet Co. I am sure there are others out there plus some local hardware stores carry rivets.
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How Sweet the roar of a Chevy four!