Olds Heads 400 > 403

supposedly 350 heads can be made to fit a 403 if the head bolt holes are drilled 7/16 to 1/2 inch. Also some drilling must be done for "cross cooling" or something

Quoting from the below link
"The stock 403 heads have 4 additional coolant holes drilled for additional cooling (because of the siamesed bores), compared to 350 heads. Drill these additional holes in any 350 heads you might use for additional cooling potential and additional protection from blowing head gaskets. This advice comes from a number of experienced rebuilders and Oldsmoholics. "

http://www.442.com/oldsfaq/ofe403.ht...IDEngineDetail

where the heck would I drill if I bought available aluminum heads ?