I answered on the other forum, but in case someone decides to search I'll post here as well.
As far as I know the flathead requires no harmonic balancer because it is internally balanced. Some guys have experiemented with them but the improvement has been very minor. There are a lot of flathead/auto combinations running around without them. Check with the guys who have firsthand experience at fordbarn.com on the early V8 forum.
You might want to consider using a C4 instead of that C6. HP usage is less than half as much in low gear.