would the maker of this 6.50-inch gun have intended it to fire stone shot?
Absolutely not, not for the big guns of the 18th C. and later.
My knowledge is mostly 18th C. and later. I think the Spanish may have used stone shot in the small wrought-iron Pedrero, which may have been in use as late as the 18th C. I would happily defer to Bob Smith on this question as my knowledge here is spotty. I'll bet Harold Peterson or Albert Manucy addressed this in their books.
The only "nonferrous" type of shot I know was used in Spanish cannon, especially in Mexico, up into the 19th C. is copper. Copper is a biproduct of some silvermines, and there were many such in Mexico, produciing an excess of copper, so the military authorities had copper canister shot cast for their guns. I expect they had copper solid shot and copper grapeshot as well, but copper canister shot has been found on at least one battlefield in So. Texas (formerly Mexico.)