Definitely call Fabcraft. The below chart contradicts the previous information and looks like it would be a series 2 carrier that is needed. Moot point until Fabcraft runs another batch of 3.55 gears. Their website doesn't state what is needed:
Fabcraft Metal Works
Here's a list of the Pontiac carriers:
1965-1970 Rear End Information
Gear Set Usage By Carrier
Year Case P/N Ratios Supported Notes:
1965-69 9784645 2.29 2.41 2.56 Except Locking Differential
1965-69 9784646 3.08 2.73 2.93 Except Locking Differential
1965-69 9784647 3.23 3.42 3.55 3.73 3.90 4.11 4.33 Except Locking Differential
1965-70 9779041 2.29 2.41 2.56 Locking Differential
1965-70 9779042 2.73 2.93 3.08 Locking Differential
1965-69 9779043 3.23 3.42 3.55 3.73 3.90 4.11 Locking Differential