That all depend on how much boost you going to go for , as you are going decompression plate would think you going for a low boost situation , and it also depends on if you going to fit 2 head gaskets ( one either side of plate ) or do the other ( not very nice ) and fit only one between plate and head .

I would think you just need to have a look around and see whats available , but would think that 2mm would be around the figure required