I have a PW3 which i am fitting into a mk2a 850 engine i have also had to open the cam tunnel to allow the cam to revolve. As i understand it this is quite common and is mentioned in Mick Hemmings engine rebuild dvd and also in fitting instructions for cam.
I have removed excess alloy with a die grinder and rotary sanding drum by hand a little at a time where needed. I am however not sure how much clearance to give to allow for expansion when the engine is hot. I want to keep the tunnel as close as possible to the cam to help with oil bath effect on start up can anyone advise on the clearance required??