Big piston calipers ?

Also 78-79 B and C. Thse castings have the hose attach point in the same spot as the 2.60" calipers.

Yes FMJ were also big bore 2.75" and use the same piston and seal kits as the B&C however the castings are different and the fitting attach points for the brake hoses are not in the same location as A,B,C. FMJ can be made to work, but with fitting location, the hoses may not be a direct bolt up.