One of the amazing functions of the Borg Warner OD system is that when the system is dropping out of OD, the electrical system momentarily shorts the ignition output at the distributor in order to ensure there is no engine torque through the overdrive for a moment.
Sounds as though perhaps the portion of that circuit that goes through the R11 kick down switch may keep the circuit shorted.
I would start with the kick down switch. Either remove and spray some electrical contact cleaner into it, or replace it. Getting the cleaner into it isn't easy, but you can also dunk and soak it, exercise it a bunch of times to make the contacts scrape each other clean.
The other part of this circuit is a set of contacts in the solenoid.
Take a look at the circuit diagram on this website and you may get more insight:
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=20645&forum=1&post_id=203036#forumpost203036 Hope this helps,
Pat
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