The 348 and 409 Mark I BBC (aka "W" motor) made more torque than the PG could handle so they all came with a four or three speed manual.
The 427 was introduced in 1966 so it couldn't have one of those installed. Since GM banned A/C with any engine equipped with a solid cam (because high RPM destroys the compressor) that leaves the 325 horse 396 which was rated at 340 horse in 1965 (because all of the BBC engines where tested at peak power in 1965, rather than off idle to lower the numbers to keep the insurance companies happy that occured from 1966 and up).
Also with the introduction of the Mark IV BBC (Mark II was the 427 mystery motor, and the Mark III was still born due to the cost of changing all of the tooling) came the introduction of a new three speed automatic introduced in 1965 the TH400. It was designed to handle the torque of a BBC.