Pinging

Dodge and plymouth used different timing specs so I won't swear but I've seen 5* even in '68. But its at something like 700 rpm not 900. Those years (68-69) the idle mix was leaner on all the engines, around 14:1.
Where I was going with this is that with the factory advance its very fast off idle. If the spec is 5 BTDC at 700, that will be very different than if its set 900 rpm.

But basically I agree with you.
* That engine, with that cam, should idle lower and use more intial timing. @FJRdoc its up to you if you want on timing curve at this time.
* Also I agree with the suggestion to disconnect and plug the vacuum advance as a test. If the ping is only moderate part throttle acceleration, then root of the problem is probably the mechanical timing is too advanced between 1800 and 3000 rpm to then have vacuum advance added in.