Most aftermarket forgings ARE "internal". Most castings are not. All of them can be "converted" using Mallory Metal (tungsten) as stated. Not cheap to "convert".
Many shops with a balancer have "on hand", stock 400 balancer and flexplate. We have a Pontiac flexplate that "lives" at the balance shop for this purpose.
The "bolt on" weight is popular among the 383 "crowd" because it allows the use of the "zero" flexplate or flywheel (they usually already have a 350 piece).
The clutch "cover" (pressure plate) is often attached during balancing for manual transmissions. Not used in automatic applications. With modern manufacturing techniques, this has become less an issue than it was 20 years ago.
Lastly, some "myths" about balancing. SAE "standards" are that the rotating assembly is balanced to within 1/2 of 1% of the total rotating "mass". In a 383, that amounts to about 28 grams. Most balance shops routinely balance to within 1 gram, FAR exceeding standards. The slight variations from using different "stock" balancers and flexplates/flywheels are not significant.
For higher revving engines (over 6,500), often "over-balance" is included. That's where the factor for the bob weight is changed from 50% (reciprocating) to a little higher percentage to offset the downward "thrust" under hard accelleration (engine speed, not car speed). High-end "big" engines usually have an over-balance of 4-5%.