"Fully balanced" doesn't mean it's external balanced. 764 is a common rebuilder's number for the factory 2801764 11" converter....normally, if the converter has external weights, the number will reflect that weight i.e. 764MW (medium weight) is for the 400/440 cast crank while the 764BW (big weight) is for the 360.

I'm betting the converter you receive will have no weights and you'll need a B&M 10236 flexplate to achieve external balance.

https://www.summitracing.com/parts/bmm-10236