Nearly Identical manifolds, performance wise. I think Mopar essentially copied the SD casting and runner design when Holley took it out of production, but it's been many years since I had the 2 side by side. if I had to run one untouched though I think I'd opt for the M1 since if Arrow or whoever developed it probably used the Sd as a mule so maybe they found some subtle improvements.

Strip Dommy or the M1 is about equal to a Victor up to about 6000 on a 340 motor and actually makes a tad more torque below 4500 (longer runners = better for heavier cars), above that RPM I think the Vic is a little stouter. For a 408+ incher I usually like the Vic cause it's easier/more responsive to porting and spacers and seems to have more meat to work with. To me, Victor series manifolds in general aren't really meant to be run out of the box...sometimes by rules you have to run an unmodded intake so in those cases I would probably lean toward the M1.