try this site
http://442.com/oldsfaq/ofe403.htm#E4...IDEngineDetail
It is from 442.com freequently asked questions section.
Also check section on Heads - will provide all that you need
From what you say your choices are, I would go with the 5's, if they are to be redone, which should be a 1969 (casting number 397742 located above center exhaust port) 350 SB head but check casting numbers and make sure they are 69's. The 7's would also work, and may also already have hardened seats for unleaded fuel
Hope that helps
John