I don't know if we want to talk about slant six designs here. But, I believe there are some notable differences and possible explanations to your experiences with it.
First be aware that neither engine has fixed ignition timing. The SOHC4 has a centrifugal advance that changes timing as the RPM increase. The timing advances with RPM increase. So, there is an idle timing as well as a full advance timing. The difference between the two is the timing advance range. The advance range for the CB750 is 23.5-26.5 degrees. If you retard the full advance setting, you also retard the idle setting Which is supposed to be 10 degrees BTDC (that's where the F mark is). Total advance at 2500 RPM is about 35 degrees.
The dodge slant six can have a number of issues regarding its timing.
1) the timing chain can become loose (stretch) and this effects that accuracy or variability of it's timing.
2) The six's timing mark is on the vibration damper which has a rubber bond section. If the rubber bond fails the timing mark can move relative to the crank's true TDC position. The result is that you cannot accurately time the ignition.
3) Some versions of the slant six were lean burn models that had a computer controlling advance timing.
4) The slant six has both a mechanical advance similar to the SOHC4 as well as a vacuum advance. At least some models do, which changes the timing relative to manifold vacuum conditions.
5) The slant six standard timing at idle was 5 degrees BTDC with some running properly at idle with as much as 15 degrees. The mechanical advance adds 25 degrees advance, for a total of 30-40 degrees. I can't find specs for what the vacuum advance adds. (probably more during cruise or whenever manifold vacuum is high.
The point is, ping occurs at higher engine RPMS when the mechanical advance is greater. If you retard the spark to reduce ping, you also retard the idle timing. When you start firing plugs well after TDC, there isn't time to burn all the fuel before the end of the power stroke, and idle will certainly suffer. This is likely why your dodge had a lousy idle with retarded ignition.
A couple other points to consider.
1) Your dodge probably idled around 500 RPM, The SOHC4 idles almost twice as fast. It would need more advance at idle to provide best "burn time" during the power stroke.
2) There is no vacuum advance on the SOHC4 to effect the idle or cruise timing.
If you are able to set your timing to Honda specs., and your mechanical advance is operating correctly. I'm thinking your idle issues are rooted elsewhere besides timing advance. If it makes you happy, then go for it. But, you may wish to revisit idle jets, Pilot screw adjustments, etc. to improve idle performance, after your timing experiment.
Cheers,
