Vintage ( LA Intake Comparisons )

When Holley quit production of the SD, Mopar Performance filled the void with the M1.
Mopar (Shepard) peddled the M1 as a superior replacement for the Holley SD. The SD has straight runners while the M1 are tapered runners. By design, the tapered runners causes the M1 to have better flow characteristics, because the 'mouth' of the runners (under the carb) are inherently larger.
Both intakes 'stand' at nearly the same Hight, so runner length are about equal.
Plenum area is larger on the M1's, and runners tend to 'meet' the intake ports at a slightly better attitude. At least on the B/RB motors anyway.
Low speed response is probably a bit better on the SD however, due to the straight runner CSA and slightly smaller plenum area.
If you need a 'closed' bottom intake, the Edie TM5 or Weiand Port O Sonic is your best choice for drag or large oval. Otherwise, an old LD340 or early Perf RPM.
FYI:
The SD I supplied a pic for is ported, and has semi tapered runners and will fit stock 340/360 ports.