FWIW, I had both intake manifolds on my 73 340 Dart sport and the LD340 ran the best. This was on a 10 to 1 engine with a .484 purple shaft otherwise bone stock. It picked up .25 sec in the 1/4 and a couple MPH. The M-1 is basically a aluminum version of an early 340 manifold. Now the M-1 single plane is a different story, When I bumped the compression and added more cam along with some head work it trumped the ole LD340

392 gen III hemi on E-85 727 trans Dana 60
10.02@134