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I highly doubt than ANY aftermarket bolt, nut, washer, fastener will be identical to those used to assemble the cars. Platings, hardening, head marking, etc. etc. can be different. Just because it is the "same size" doesn't make it identical.
And, if you don't really care that it is "identical", the same size will be fine. I tend to give bolt heads/nuts/washers a shot of engine enamel clear paint on them before I install a bare-metal item to my car. Why ask for more rust??
I like going to the local pull your part wrecking yard and picking out a GM car from the 70s or early 80s. Grab you a coffee can and spend about a hour pulling different size bolts and screws from a few vehicles.
Theses are the original style bolts and screws GM used and look much better tan the china bolts from the hardware store.
Much cheaper also. Expect to pay between $5.00 and 10.00 for a coffee can of bolts.