The seal does not need to move much, just enough so the lip can not fall in the existing groove which will cause it to not float free to the surface as the crank moves about. .050 to .065 is plenty. The face of the old seal might be about right.
The 2.6 heads are not prone to warping, they do have an issue with the head bolt settling in which reduces the torque on them generating gasket failures. Re torque them periodically, use every other oil change which may be too often but I have had failure free results. If the heads have been tormented during there life they can crack in a variety of places. The earlier alloys are tough to weld but this is one place the aftermarket heads shine. I have not even considered trying to fix a damaged head since the Marnel heads came to market.
A nice carpenters level can provide a straight edge.

Thank you for the kind words, I have learned allot over the years an lately to offer respect for those that passed on much knowledge to me I like to share mine with those that want to receive it. The time I have spent learning does no good for others if I can not share it. Thanks to these forums I can, plus I still get to learn more too.