[bigblockv6]

As far as I know that was the only major difference, the 305D came out mid year in 61 and it has been said many 305A's had been switched over to the 2bbl at the dealers in 61 because of the poor performance of the 1bbl. Net horsepower for the 305A was 125hp versus the 305D at 142hp so that is a 17 hp difference.