[David Hodgson]

Cheapsnake
Unlike performance engines where you balance your internals and externals, Ford on their production engines used weighted externals, H/Balancer and flywheel to achieve a reasonable balance, hence the term you hear - "external balance".
They've used two different weights - 50oz and 28oz. You will suffer vibration problems if you mismatch the flywheel and h/balancer unless of course during your build you have done a complete balance.