OK, had to look it up. Hemmings says 290 hp for 73-74 SD 455, Car and Driver says 310 for the 73 version. (They reported a quarter mile time of 13.75 at just under 104.) Without doing more research, I'm thinking 73s had 310 and 74s had 290.
Still decent for 8.4 to one compression. If I remember correctly, a 440 6 pack was rated 305 net hp with 10 to one CR.