I have gone both ways in the restorations, that I have done over the years. I have used original bolts and sent them out for replating. I have media blasted originals and used restoration products (like Eastwoods products) to try and achieve original finishes.
For several years now, I have been buying from AMK Products in Winchester, VA.
www.amkproducts.com . I also have a body and trim shop supply house here in Denver, that carries some fasteners. AMK starts in 1964 with their kits, but they have many products you can use to do a "correct" restoration. You will notice in studying their catalogue that they lean heavily into Ford products. AMK still sources from many of the original Ford, Chrysler and GM suppliers.
I thinks it is easier and less problematic to use new body bolts if they have generic markings. GM used numerous suppliers for fasteners, and different assembly plants will have different markings for the same applications.
When I restored my 1967 Chevelle SS396 convertible, I spent a considerable amount of time studying what was correct, and what wasn't. It's amazing how much misinformation is out there. Luckily, the Chevelle model has a rather large collection of experts who have helped document the originality of this model.
FWIW, I have a friend on another forum who is considering making a video (or CD) about the correct restoration of the 64 model. I am trying to get him to expand it to cover more X frame cars. I know that many of you could care less about correctness and restoration, but I personally believe there is a place for it, in preserving automotive history. I'm not trying to get on a soap box here, but I love survivors, hot rods and restored classics. "Restoreum, don't crush them."