Ah yes, that's the A12 crossmember. Someone once said most B310 sunny wagons came with A14.

Other than fabricating a 56-to-60 crossmember like we do for 1200s, your options include buying one from the wreckers, or asking if a club member has an extra one for free. It's possible you move the 56 crossmember around (there are three sets of bolt holes under the car) and use a spacer and/or adapter plate of some kind, but I've never heard of anyone doing so.