Frank,
The 235/261 are natorious for cracking in the rounded over area between the intake valve and the chamber. While your proposed groove location doesn't go directly through there, it is right at the edge of that area, so cracking may be more of a possibility than you think.
Second, it would seem to me that for this groove thing to be affective, it needs to be relatively centered in the squish area, but that isn't possible on these heads. Your groove is about 3/4 of the way over to one side of the piston, so I'm not sure how much is to be gained from it.
Lastly, how are you going to know if its done any good?
