Matt, there is some chatter on this very subject over here.......
Cylinder head part numbers
Perhaps they can help with a decent answer.
Cheers,Steve 
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1981 911SC Targa-1966 912 -1989 Alfa Spider Graduate
1967 912-1985 Toyota FJ60 Landcrusier
1985 Toyota SR5 4x4-1965 Baja Bug-1997-4Runner-4x4
1966 Bug stock-2004 Toyota Rav4-1989 XJ6 Jag
1975 914, 1965 Norton N15CS 750, 1975 Husqvarna 360 CR GP 1982 Honda 500 XLS
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