Also need shorter stainless bolts, the threads ended 1/4" too soon and I have multiple washers just to keep it tight.
You might want to get some heavy SS fender washers/spacers made but that is what washers are for, in reality there should be no threads in the transom as threads provide less bearing surface than the solid shank and can allow movement which will lead to more movement. Think of the bolt shank acting as an alignment pin and the nut offering compression to the joint.