Harmonic Balancer Question

On big block mopars we all know there were balancers specific to either a internal or external balanced motor. When purchasing aftermarket parts, I dont see this option. I acquired a 400 that was built at some time and it has a SFI balancer they resemble the size and shape of an internally balanced motor. How are these interchangeable? Just curious for self wisdom.

Thanks

lol question answered.... This is the unit on the motor:

https://www.manciniracing.com/cfvupload49.html

Like you found out, some have bolt-in counterweights.
Also, check if the flexplate is counterbalanced, or the converter is.