10" brakes on formula S cars?

Daves69 said
FWIW, after going to disc I gave my 9" drum set off my '69 S (383) to a member here to make some space. Sold the LCA's (with sway tabs) to another.
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That's even more crazy. 9" drum on a 383 car? There has to be a option or reason why. Doesn't make sense.