The rule has been 7k for about 20 years.
What are your wild cam specs? What brand of piston do you have? How far is the piston in the hole? You will need to mock it up with a degree wheel, checking springs and a dial indicator to see where your at. Then you need to measure the piston deck thickness to find out how far you can cut before you start cutting. Sand paper will just plug up. Unless your piston is way down in the hole and it shouldn't be for a race engine, It's not likely to have a thick enough deck. Also if these pistons are cast and not at least a hyperutectic performance piston (which would most likely already have valve pockets in it) there is little chance they will hold up to racing at 7000 rpm. for any length of time. Give us more information about your build please.