Regarding the front drum to support bushing - there are two reaction shaft support bushing sizes from 1962 - 2008, narrow and wide. The metal seal ring spacing never changed on the wide bushing support.
I think you can install the wide bushing drum on a narrow bushing support and the unit will assemble - that is not suggested and I cannot see how that could cause a leak like the op has but who knows. Good catch.

There is absolutely no reason to change the vent location on a non-transbrake application.
Millions of vehicles without an issue. And a bunch used in racing without issue too.

I would start with the oil level verification suggested. If that doesn't produce results then time to get deeper.

The Sonnax regulator valve is not needed either. A pump with proper internal clearances will prime the system and fill the converter in a few seconds. It's a nice addition if you want to spend the money but it's not going to fix this transmission problem.