If you have an October 31 build date on a '67, your car was built October 31, 1966. You are correct on the year.
In talking to people who were working at Pontiac during the 60s, the concensus is that inventory control of parts staged for production (like block castings and head castings) was, at best, poor in those days. Although it is normally recognized that casting dates for parts on a car should be a week to 2 months prior to the build date of the car, inventory control issues often resulted in casting dates outside this date range even on "correct" cars.
Also, note that many of the foundries did not use the letter "I" in their date codes (just like good engineering practice is to not use the letter "I" for drawing revision levels) since it can be mistaken for the number "1." For this reason, you can see the letter "M" for December, even though it is the 13th letter of the alphabet.
For an October 31 1966 build date 67 GTO, you should be looking for a block casting date of August, September, or October 1966 ("H", "I", "J" and even "K" since "I" was often not used).