Well, you can but jeez. Its not the end of the world, you'll need to really tape off the entire top of the head except the area being clearenced, and I mean tape it off, use a good quality duct tape, not the cheap junk. Make sure its clean and dry. Buy a small fine file (round and flat), I filed because i didn't need all that aluminium floating around from a grinder.
I've done it, in the bike, so it can be done. Use a sharpie and mark off the area in question so you know exactly where and how much your removing. Do one side at a time (I did the right), all mine were on the front (exhaust) side of the head, looked just like yours. Finish one, vacuum it out and check it. Put the cam in and and check the clearance, I think I made .015 possibly .020 between the lobe and the wall. If you need to, pull it out and file it some more. The only area not taped is the trough the oil gathers in under the cam, that's where your filings can collect to then vacuum out. The after pic is hard to see but its done. Keep it taped and clean, one side at a time.
