Question about ignition timing on 327

[Metalaroundstone]

Attachment 1187607I have a question about the proper timing spec for the 327 engine in my 1966 c10, which has a 4GC carburetor and no air injection reactor. Right now I have it at 8 degrees BTDC, with vacuum advance disconnected and port plugged. However, when I look at the specs in the service manual addendum for 1966, it says the timing should be set at 2 degrees BTDC. The 1963 service manual, on the other hand, says it should be 8* BTDC. My truck doesn't seem to like a 2* BTDC reading. I would appreciate any advice any body has on what the ignition timing ought to be and why these two spec sheets are so different for the same engine?