66 Valiant Signet has a 360LA engine - what do I need to know?

A cast crank 360 definitely needs the external balancing. Unless your bro converted to neutral balance.
The weight in the front was cast into the balancer. Can't remember how it was done on the flywheel.
Mopar Performance/Direct Connection used to publish the drilling required to convert a neutral balance FW to external. Maybe someone here has access to it.

Having said that, and reading your other post about this car....I'm sure your brother knew what he was doing and did it right! Especially if he raced it and worked in the industry all those years.