Ball size in that one, smylee G would for me, depend on the depth of the rifling. If typical .012" per side, a ball from .005" to .010" would be my choice if buying from Tanner moulds, so .650" to .655".
I think there might be a .648" in standard size and that one for me, would be a likely candidate.
Now, if you were wanting to use hard lead, that suggests another option in mould size and a .645" mould would be just fine, just as the .677" (casts 675" in pure) works in my .69 with hard or soft led, even with the .034" denim patches.
I think dropping 2 bore sizes in ball dia. is not a good choice.
That said, I did shoot a group at 50yards with my .69(14bore), but using 2 .017" or .018"(8oz denim) patches & a 16 bore ball (.662") the group turned out good enough for hunting.
I had a flier(flinched shot) that threw well over on the right, which is typical for a right handed shooter's flinch. Thus, I fired an extra shot into the group for a total of 5 shots in the group.