You would definitely want to get a good grade 8 head bolt. I would try Mancini racing, summit, or any of the fastener suppliers here that make reproduction bolts, ARP, SPS etc. The head bolts are 1/2" coarse thread. I believe 1/2" x 13 threads per inch.
Figure out what length you need as I described in my other post above and then look for those lengths. Remember one of the "long" head bolts is longer than the rest as it has a higher "boss" in the head. Make sure that you check each hole in your head (pun intended) to make sure that you have the proper length. Maybe draw a "map" of the bolt pattern for your head on a sheet of paper and number them. Then under the diagram list 1-10 and then use each line to record what length/depth bolts you will need.
