The 300 SE is a good "town car", and I think the second gear start on most is what makes them sluggish off the line. Maybe there's a way to modify the second gear start to always start in first.

The M103 has a timing chain, not a belt, and it should last for nearly the life of the engine. The thing that wears timing chains is dirty oil.

Removing the R16/1 resistor increases spark advance faster with revs, so low end torque is better.

CA has labeled the M103 as "high emitter profile", not a "gross polluter" unless the subject car exceeds the gross polluter cut points, and all will likely be directed to a "test only facility". I have one less than a mile away. It's a "drive through" type and they accept coupons, so the typical cost including the "smog certificate" is about 50-60 bucks.

Emissions will be much lower if you short the R16/1 resistor connector and disable the vacuum advance. I did extensive testing on this setup and documented the results here in a couple of threads about 10 years ago. Search for threads started by me and R16/1 resistor.

BTW, there is a CA Web site where you can input the license number and get the emission test history in terms of pass or fail. I think it might be the Bureau of Automotive Repair site. They manage the CA emission test program.

The 1:1 top gear and 3.46:1 axle means high revs as stated at freeway cruising speed, but the engine is very smooth at speed, and has a pleasant muted "growl" that is typical of high output inline six-cylinders.

M103 idle smoothness is an issue. Be sure to use non-resistor spark plugs. Resistor type will roughen the idle. Also, I think the KE-Jetronic fuel system may be an issue since idle fuel pressure is low, which causes uneven fuel distribution compared to modern EFI systems.

In addition, the M103 has a pretty aggressive cam in terms of duration and overlap, which is what makes it so rev-happy, but manifold vacuum is relatively low at idle, especially in drive with an automatic. Any engine that idles at about 14-15" Hg or less manifold vacuum will exhibit some "lope" due to valve overlap - similar to sixties vintage Corvette engines like the 327/350 HP engine that has a "hot" hydraulic lifter cam. It's idle behavior is 14-15" @ 750 in neutral with a slight lope.

Duke