Kubota engine gaskets are designed to go on with nothing but a clean, dry, flat surface-and nothing more. Sealants-even Copper Coat, have been known to develop issues in the long run, in my own experience.
The shim is also used to set the clearance between the piston and the head. Some of them need it, others do not. Your service manual will specify exactly how to check it. If it's put in and doesn't need it, compression pressure suffers. If it's not put in and it needed it, there is a chance that the pistons will contact either the valves or the head.