Its pretty difficult to get a rigid setup like that. I do a lot of this stuff at work, usually only the turbo face though (head face normally gets done on a belt sander.
Some things to try;
- You can try and take really small cuts in that area with a freshly sharpened tool and file and emery it to make it blend into the rest of the face. As long as it is flat it shouldn't leak.
- Chamfering all of the edges works well for keeping the tool sharp and cutting nicely as does a bit of cutting fluid (wd40 or kero are fine).
- Using a lead hammer or similair (dead blow) and rest it underneath the flange while the tool is cutting on top. You may be able to soak up the vibrations enough to get the finish looking alright.
- If its just that section, stop the cut before it reaches that part and add some clamps to the flange where you have already passed.
Most of this stuff probably wont work terribly well. If you plan on doing a few of these, I'd suggest investing in a large vice and an angle plate, they can come in handy for a lot of jobs.