different sound to it than any other /6

there are three foot long extensions after the Dual Dutra's. That then Y in together.
I have been told the exhaust has a snarl to it. The motor was not built / cam'd for RPM
The exhaust tone comes on in the mid 2000's RPM

Interested to learn about this exhaust design. I am guessing you built it that way for torque-or low RPM response?

Exactly, for the exhaust I was most interested in scavenging.
The down pipes are not large, 2 1/4 to keep velocity up. After the Y it goes to 2 1/2


A portion of the exhaust tone is also due to cam lob timing.
I did something unusual on the cam grind. It was ground with the exhaust center line at 105, that opens the exhaust valve early. Normally exhaust center lines are 110+. Again I was after exhaust velocity and emptying the cylinder to help fill when the intake opens.

I distrust any recorded exhaust noise, such as the example in the video. Or when someone posts a video/audio of their new Flowmasters, etc. Too much opportunity for the recording to change the actual sound or pick up/ignore noises you'd hear in real life.

I think that Mercedes sounds the way it does for a variety of reasons...exhaust system/size, intake, cam timing/overlap etc. I have no idea what its engine looks like but I'd also bet it has a crossflow head and multiple carbs.

I don't think sound is a bad reason to build or change an engine. Just look at the exhaust aftermarket. Heck, Ive been eyeing the Mercedes-Benz M120 V12 engine for just this reason, but will probably settle for a high tech I6.

You mention idle twice. I don't hear anything very distinctive about the idle. Maybe just a little valve train noise. Maybe. What I do hear is pronounced intake roar. The 300SL has a long runner manifold and I think you're hearing intake pulses. I would want to see the intake ahead of the throttle body, but it sounds like is has an un-silenced air cleaner.

That's an awfully expensive car for that kind of work. I'd use a much cheaper, and newer, BMW rather than risk the 300 SL.

Here's a 1953 Hall-Scott 400 series engine of 1091 ci. https://www.youtube.com/watch?v=po_Y7EspkUs

So your intake lobe is of close to normal interms of duration and intake opening/closing,

yes the intake is on a 105 centerline, and then I advanced the cam a couple of degrees at the install.

and yes the goal was to clear out the cylinder with a lot of exhaust velocity at low to mid RPM's.

Did it work, well I think so, the truck pulls strong starting in the low 2000 RPMs, but I have no back to back data:
same engine, same vehicle, different cam. I wanted to try an idea and I did.
I picked up a small NOS Mopar Performance slant cam and I plan to install that eventually, just to see, hear the difference.
I may get to that this year,,,,,

I will get you get you a sound clip when I get the D150 out of hibernation.

That's an awfully expensive car for that kind of work. I'd use a much cheaper, and newer, BMW rather than risk the 300 SL.

Here's a 1953 Hall-Scott 400 series engine of 1091 ci. https://www.youtube.com/watch?v=po_Y7EspkUs