Try calling around to some local plating shops and see how much they charge for zinc plating your bolts. That's the original plating on the majority of the parts used on your chassis. The Grade 8's were black oxide finished to avoid the hyrogen embrittlement issue mention previously. You'll be surprized how inexpensive it is to have zinc plating done. The shop may also be able to remove the rust for you. I'd replace the grade 8 bolts with new just because they were used in highly stressed locations. Paragon Reproductions sells "correct" bolts with all the original head markings. They also have a chart available that tells which bolts were used where and what the original finish was.

I use Captain Lee's rust away to clean some rusted hardware. Soak the rusted parts in it over night and it totally disolves the rust and leaves the base metal alone. http://www.halonmarketing.com distributes it. The stuff works fantastic!!

 

I wire brush my bolts and then run them through a die. I also tap nuts. Eastwood also makes a black phosphate simulation kit to recolor the bolts. (It's really not the same as the industrial process, but it does use an organic acid to surface treat the metal black). Incidentally, there's a good reason to reprocess some of your frame bolts - that is I don't think you can buy replacement bolts. For instance, the lower control arms are held on by three bolts; two "little" ones up from and one "big" one facing the firewall. The big one has a shank that is about 10 mils or so larger than the shank of commonly available bolts. This bolt not only acts as a "bolt" it also acts as a pin. You lose the pin functionality by buying a standard grade 8 bolt.

Also, the trailing arm bolts, of course, do not function as bolts. They are pins. They have threaded portions just to keep them from falling out. If you buy new trailing bolts from the Corvettes part houses you will find that they/ve just machined a standard bolt. When you insert these repro bolts, the threaded portion will push up against the frame. The threads will cut into the frame due to shear forces. I've decided to buy extra long standard bolts to achieve the proper shank dimension, and then cut off the excess threaded portion. I've already done this with my differential bracket bolts - they the same bolts used in the trailing arm and these bolts also act as pins not bolts "per se." I noticed Twin Turbo's comment about useing Grade 5. I used Grade 8 and now I think maybe I should use the Grade 5. In this application, maybe the hardness of Grade 8 is slight disadvantage.