As above , why do you want to change them ? They are not known to suffer problems on the KV6 so unless you are doing other head work like valves or valve stem oil seals i would leave them alone .
If you are suffering oil smoke on startup then it is worth renewing the valve stem oil seals whilst you have the engine out .At just over 100k miles my seals were rock hard and not sealing much at all
The heads do not necessarily need skimming unless a straight-edge check reveals them to be warped ; this is unlikely . Mine were fine
After the gaskets are removed however you are likely to be faced with a very dirty face which will take some cleaning . I used a piece of 4x2 timber which had been machined flat , wrapped in a sheet of fine grade wet and dry paper lubricated with soapy water , to polish up the face . Made a very nice job .
You can reuse the bolts .