Base timing. Really ?
Yes that would be the approach.
I don't have any good info on late smog Chrysler distributors.
You may find the smog advance curve isn't quite as extreme as the '68 440 with M/T
It may have other differences too but it will still be a two stage advance.
I know with AMC Jeep V8s, in the early 80s they were able to run more normal initial timing and advance. I suspect this was because they were able to advances in catylitic converters and other tweaks to offset slightly higher idle emissions.
When take the distributor apart enough to get to the advance plate you can see how much advance is in there. The plate (aka governer) will be stamped with the approx degrees. Also take all the measurments with a caliper so you have your own reference.
You can also play with the moving the weights using needle nose pliers to get sense of how many degrees of movement before the long looped spring engages. A few guys have used a degree wheel and a homemade pointer to measure movement (distributor degrees) on the bench.
I know this is taking a lot for granted, but I would like to be as close as I can to start
Sure thing. Might as well take advantage of the research and testing Chrysler did for their engines and vehicles.
The most difficult part in the disassembly is removing the spring under the felt.
If you think idle is going to be around 900, probably best to start without weakening the tension on the weight.
If the weights start moving out at 750 or 800 rpm and the engine is idling around 900 rpm, timing will change with every adjustment which then changes the vacuum which effects the idle circuit and in turn the rpm. Its like chasing a moving target.