Thanks motorgeek. There are a few experienced machinists around so I'll be able to sort that out easily. It's interesting that you're able to add centre gaskets though.

Glamisrider, I'll just cut them myself from gasket sheet which will give me a bit of freedom to take off just the right amount of material from the cases as the sheets come in different thicknesses. I've made a center case gasket before from 0.6mm sheet without a problem.