Unless I'm wrong I thought the most the 420a cylinder head could be surfaced till the valves started getting too close to the piston at tdc was .040 - .042(even when timed correctly). When I was working at a engine rebuilding shop I did valve jobs and never heard of any head getting decked that much, let alone a 4 cylinder aluminum head. I wouldn't use it because any kind of warpage to your head would not be good, especially in your situation imo. The most I ever shaved off any type of cylinder head was mine at .021 and I did it to raise compression.