ASE Certified Master Machinist
1967 Chevrolet Biscayne 10-71 blown 433 Big Block Chevy, T400 & 12 bolt 3.73 rearend
If it is a standard cast iron SBC head, the valve guides are integral. They are not replaceable. The old guides can be bored out and new replacements installed but this is not something you can accomplish with hand tools. Take them to a machine shop and have them replace the guides. That is unless you happen to have a seat and guide machine such as a Peterson TCM 25 handy. Good luck with your project.