Editing Drilling steam holes in smallblock Chevy heads

guess we're just trying to make sure don't screw up cust. brought [new] pr [aftermarket] sb heads cust. brought [old 400sb chev] head gskt . wants us to lay head gskt on blk surf. side , mark ALL steam holes , then drill ? is there ANY thing we need to sure of before we start drilling or do we just drill , then give back to cust. [we've been in the eng. rebuilding business [33+yrs] BUT , as U can tell this is the 1st time anyone has asked us to drill steam holes ????


==Solution==
You should use a 400 head gasket as a template to mark the heads and Drill the 3 holes on the intake side using a 3/16 drill bit at a 10 degree angle towards the exhaust side of the head. Drill the 3 remaining holes using a 1/4 inch drill bit straight down towards the top of the head. 
1- Start by marking the approximate steam hole locations with a marker or die. Next insert 2 dowel pins into the heads dowel pin holes and lay the 400 head gasket onto the head over the dowel pins. This will keep the gasket aligned precisely as you mark the holes.
2- Level the head with the deck side facing up and drill the 3 holes that run parallel to the exhaust side of the head using a 1/8 inch drill bit. The maximum size for these holes are .250 or 1/4 inch.
3- For the angled holes, level the head at 10 degrees so that you will be drilling from the intake side of the head down toward the exhaust side 
4- To drill the remaining 3 holes in the head use a starter bit first then switch to a 1/8 inch drill bit. This will make it easier to   get the holes started on angle without drill deflection. The maximum size for these holes are .187 or 3/16 inch.
5- clean off heads with brack clean or parts cleaner. use a new head gasket to make sure holes are correct.