360 la weighted flex plate do I need to rebalance?

Define "balanced". Is this still yet an externally balanced engine, or did you have it modified for "neutral" balance. There is room for a lot of misinterpretation, here.

EG If it's a "as stock" external balance engine, and you had a shop "balance the assembly" as such, it would have had to be balanced with the weighted balancer you are going to use, and the flywheel/ converter you are going to use, (or a weighted BM style flex plate for use with neutral converter)

On the other hand if you had it modified --as with Mallory Metal, etc---to become a neutral balance engine, then you want all neutral components, ALL--balancer, converter, flywheel, flex plate etc