I'm trying the same thing, I put the c20xe head gasket on the z22xe block last night and dropped the head on, double checked all galleries and ports lined up ok. I'm going to use the z22xe head gasket and c20xe head. Only thing I'm not sure of yet is the clearance between the valves and the pistons, but I think it will be ok as the c20xe pistons sit slightly proud at tdc whereas z22xe pistons recessed into the block.
You MUST do a dummy build with plasticine, or similar, to check the actual valve to piston clearance as, I believe, the valves are placed slightly differently in the two engines.
The conversions, like Alex's, that I know about used XE pistons.