With an electronic, start with a 13 or 15.
As the weight moves out, more spring force acts against the outward motion of the weight. This slows the advance, gaining a degree or two in the top end or at least offsetting retard from slew rate.

Red arrow is force of the spring acting on the weight.
White arrow is acceleration force of the weight on the spring.

Let's take that and see how Troy, the OP might apply that.
1967 specs are helpful because they show us the curves before they started manipulating things for emissions - especially idle.
If the 426 distributor in use already has a non-CAP advance curve, then bumping initial to 14* or even 16* may be fine. This will depend on the fuel and specifics of the engine, as well as use. Drag race only will tolerate quicker advance than an engine that gets really heat soaked. The advance can be slightly delayed by putting a little more tension on the primary spring.

If the distributor has CAP type advance, its going to be more work. It may be possible to simply move the secondary spring perch so that the long loop engages around 5* out instead of 12* out. Maybe that's what the factory did, or maybe they used a different spring, and/or a different cam plate. Only someone who has had these distributors apart and looked for these things will know. From the specs we can be pretty certain that a higher rate primary spring was used on the non-cap versions. The tension was adjusted so they both start moving just above idle.

This graph is in distributor degrees and rpm:

When these distributors were installed and set to the specified initial timing, the off-idle timing curves were very similar. Ford service manuals explicitly advised that the timing could be increased in 2* degree increments for better performance, or reduced a little if fuel or other conditions required. I've not seen that in Chrysler documents, but I suspect the same applied.

Notice below that setting the initial for the non-cap distributor at 14* will result in something close to 34* at 2800 rpm. However doing the same with the CAP distributor wouldn't work out so good unless the secondary spring was made to engage at 5 or 6* distributor degrees. That could be done by welding the inside of the slot, or with a different spring or by bending the perch. I can't say which might be best without having one apart and measured. Even then it can take a few tries. As always, someone with experience doing them will save time.

This graph shows timing, its in engine degrees and rpm.

i have such an animal that would work perfectly for this. i made it a ton of years ago because i got SUPER tired of always juggling around with 2-3 mini screwdrivers to remove that spring. anyone out there willing to guide/help me set up a sellable item ? i'm old, and not literate in patent processes, plus, i'm not up on marketing that would be for an item that "may" have limited interest. then, there is the possibility of showing this item and having it ripped off, making someone some bux, while i get no credit.

A pair of extended snap ring style plier with outward facing, small concave tips.

I use a pair of thin needle nose pliers to spread the vertical legs apart (3 or 4 tries) then when it is expanded as much as it can I grab on one of the legs & whip it around to the open side & up all in one motion. it distorts the clip a bit but they are very non brittle (I forget what the word is for the opposite quality) & will easily withstand this. sometime in the future I'd like to thread the shaft for another type of holddown.