In the articles I read where Steve Dulcich did dyno test on 360 with several intakes, the M1 beat the low rise dual plane, but did not beat the RPM airgap. The M1 had suprising torque at low end, for a single plain. I remember the article well.
That's the only one I can find on Mopar Muscle done by Dulcich. It's the main page where it shows the results of the test but it doesn't show an M1. Rather it shows a Holley strip dominator which is nearly identical to the M1 I've read in several places. Feel free to correct me if I'm wrong about that. The test was on a hot 360 with a lot more cam than you have so really the M1 would be much more inclined to work better on it than a mild 318. Yet in the test results it wasn't until 3,800 rpm that the big single plane overtook even the std. performer. AT 3000 rpm the performer was 22 ft. lbs ahead of the single plane in the torque column. I have never used an M1 but I have used a few different single planes over the yrs. and the general experience I've had is you need at least a 3500 stall minimum and low gears to make them work any better than a good dual plane like the RPM on a small cube engine.
BTW: What makes you so certain it's the intake that's causing it to fall off at 5500? I don't doubt it has something to do with it but as I already stated I think it's more the cam. You then stated Dusterb318 runs a cam about that big but remember this, his is a solid. A solid will pull quite a few more rpm than a hydraulic. A good example I know of personally is a buddy of mine runs a 340 (same stroke as 318) with a Comp solid that's 242/252 degrees at .050 and .520/.540 lift and it runs best shifted at 6600. Sure it'll spin to 7000+ a little but it's not making any usable power and goes faster shifted at 6600. He also has nicely ported X heads so it can breath. I've seen examples of that time and time again. Just my .02
