Hi, the best match for a replacement shank, for my set up, was an 18" shank. I had a new hole drilled 2"s back. Later I had another hole drilled 2"s back and the excess end cut off. So my shank would fill the length of the receiver, the first time it was re-drilled I had 1" cut off of the end. When I had it re-drilled a second time, I had 2"s cut off of the end.
From hole to hole, if I could find a good match, a 14" shank would work. Mine ended up being 15"s because I wanted the shank to go all the way into the receiver.