Every thing I can find on the web lists the standard engine on a 71-76 Bel Air or Impala as a 155 HP 250 I-6. The Caprice came standard with a 350 V-8 those years. I have seen a few I-6 powered 71-73 Bel Air's with the I-6, one was even a 3-speed manual column shift. I don't think they sold many so equipped any of the 71-76 model years, and probably fewer as the models grew heavier, and the HP ratings of all the engines dipped in the mid 70's
If I remember right, the 250 made 155 hp in the sixties. You would think that it produced less later on, being smogged down. My dad had one in his '67 F-85 and it pulled it pretty good for a 2-speed automatic. It would be insane, to drop one into a heavy bumpered late model barge, though
