Far as I know they`re external balance on a 454.
If a non external balanced job was installed it wouldn`t have ran very
long before it destroyed the thrust surface. One time when I happened
to be in the same place where a guy swapped on a new flexplate on
His 400 small block that was a 350 unit, he got out of the driveway and
about 60 yards away before it started knocking.
In this case do you mean flywheel as in for a stick shift or flexplate for a automatic?