I'm thinking of upgrading my R33 GTR camshaft to 272 stage 2 with valve spring. I've heard that several modification has to be done. Maybe head shimming or something with the valve / shim the tappet as the lift is higher. Please advise. As i know for stage one camshaft (272 also), i've installed into my friend's car without any modification on the head and no problem to date. Any advise is highly appreciated.