The base horsepower was raised to 275 in late 1965. It was the same engine as the the 250 horse motor except for the replacement of the Rochester 4GC with a Rochester 4MV Quadrajet (which was rated a 620 cfm to the 4GC rating of only 480 cfm). Because the Quadrajet had a smaller primary and a set of variable venturi rear barrels it could not be over-carburetored; as the fuel was metered for the amount of air flowing no mater what vacuum the carb saw (it beats both Carter and Holley carbs that have a fixed cfm rating in this regard). So if the engine needed the extra cfm to make power it was available hence the higher figure for the base motor.
The first production year QuadraJet have a unique feature that isn't found on 1966 and up models of the carb; in that they had a secondary throttle shaft lock out that prevented the rear barrels opening unless vacuum drooped enough to activate a second vacuum module that kept the barrels closed until the motor was screaming for more air. People complained that the "passing gear" was engaging way too rough and threw them in the back seat when the carb's back barrels engaged.