It is a '67 327, hence most likely a restamp as it'll be a small journal block. HK 327's with 32712xxx will all be 3914660 and just 4 x possible assembly dates all from Tonnawanda plant, Pontiac and Impala Pagewood engines have other assembly dates but the same cast number and Tonnawanda also.

Very unlikely. It is possible I suppose GMH tried one in an early HK but it wouldn't have an engine number like 32712xxx. Remember all 1968 Pontiac and Impala assembled by GMH in 1968 (like 6 months or more prior to Monaro) had large journal 3914660 blocks with engine numbers amongst the HK 307's so they were in widespread usage a long time before Monaro production. The first HK Monaros from Pagewood actually had enigine numbers around 30712xxx and 32712xxx.