The 6 & V8 wiring is the same except for the 2 extra spark plug wires. GMH didn't issue a separate diagram for the V8.
You are correct about the number referring to the wire gauge, although I don't believe that GMH did this for the LH diagram.
You say they didn't issue seperate diagram for V8. My diagram says to refer to sheet 12zz-2 for 4 and 8 cylinder engine wiring, so maybe just an extra page showing only whats different. Do you have an idea what this 12zz-2 sheet is.