Need a Harmonic Balancer. Which one?

Having some work done to my (113,000 original miles) '71 Dart GT. It has the original 318 V-8.

My mechanic buddy is installing SummitRacing double roller timing chain and gears, LD4B Edelbrock intake manifold, 1406 600 c.f.m. Edelbrock carburetor, Cardone 8-vane water pump, Lokar "kickdown" and throttle cables, hoses, fuel filter, radiator and block flush, etc....

He told me today that the Harmonic Balancer has crack in/on it and it needs to be replaced.

Looking online I found a couple choices at SummitRacing, but I'm unsure as to what I need. The car is never raced (just a ice cream parlor cruiser). So I guess a stock replacement is all that is required. Do I need internal or external balance? So many choices. Any recommendations/guidance is appreciated as always.:thumbsup: