Need advice on porting Performer RPM heads

I won't personally be doing the port work but I was just wondering if there is anything in perticular to watch out for or any tips you guys may have. I will need to take about 1/4" off of the walls to mate up with my Kooks headers. Maybe you guys could suggest a good CNC place I could have it done. Maybe there's one in Nebraska?!?!

Thanks

Mike