Beginning in 1970 any "A" body with a 455 got a 12 bolt. This included the Grand Prix. This existed through at least 1972. I had a Grand Prix (1970) w/455 and it had a factory 12 bolt. I bought the car in 1972/1973(?). GP was a stretch version of "A" body.
For "F" body cars beginning in 1970 some 400's came with 12 bolt and in 1971-1972, some had 12 bolt and some didn't, even with 455HO. Friend had '72 T/A and it did not have a 12 bolt. Have an article with '70 Formula 400 w/12 bolt.
Who knows why they did what they did with "F" bodies.
