340 internal balance or not?

I am putting together a 71 340 with a later cast crank. Is it necessary to have internally balanced or can I use the cask crank harmonic balancer and use the B+M flex plate for the 904/340 combo?
I understand there is an adapter that needs to be used with the early 904 and 340 combo. I remember reading about that earlier, but cant' find it now.
What do you think?
Thanks for your help. FABO folks are the best!