If you haven't done a couple dozen sets of heads before, then I would advise you not to go grinding on the heads. You might accidentally cut in the right places and add a little flow, but you might just as easily cut in the wrong places and turn them into junk.
I'm gonna tell a story on myself. When my youngest son was in high school, we built a '72 Chevy Luv pickup with a California Stepside conversion. We shoehorned a bone-stock 455 Olds into it, with a TH400 and 10 1/4" Olds rear diff. Took it to the dragstrip and turned 101 mph. Pulled the heads off the motor, spent the next 26 hours sittin' at the bench with a Dumore and carbide burrs, then sanding rolls. Bolted the heads back on the motor, went to the dragstrip and turned 103 mph. Picked up a crummy 2 miles per hour in return for 26 hours of labor. Now, someone who knew what they were doing may have been able to pick up some real power with those heads, but I was not that somebody and it is doubtful that you are that somebody either.