The biggest thing on what octane you need is what compression does your engine have?If your down around 9 to 1 you can get by on the cheap stuff. Get up to 10 to 1 you better get the 91 octane. Up around 11 to 1 you better be mixing in some racing fuel. Also backing down your timing will help if your engine starts to ping.
69 Mach 1 427 Windsor,T-56 6 Speed,4.56 Traction Loc