340 internal balance or not?

Yes, you can use the cast crank balancer and appropriate B&M flexplate, but it all depends on the weight of the rods & pistons. If you are using aftermarket or the original '71 pistons, you should get the overall balance checked anyway. Different pistons can have a different weight than the original low compression versions used with the cast crank and can throw the balance off.

An adapter ring is only needed if you try to mate an early 904/converter (up through '66 or '67?) to the later cranks because the converter snout is smaller than the corresponding hole in the crank.