Actually.... it's balanced. Balanced for the 360. Read your advertisement for the B&M flexplate. It's called externally balanced. :) Other wise it's called internally balanced.
Like the early 340 crankshafts (from 1968 through 1971 model years) --- the flex plate required for a 727 attached to an internally balanced engine like the early 340s is an internally balanced flex plate.
Take a look at the crankshaft damper --- internally balanced cranks have specific notch markings noted on the damper.
Externally balanced cranks use dampers with add-on weight slugs that are easy to spot.