what are the clearance specs for porting 350 heads?

just what the title says. my friend is wondering what the specs are for porting a stock set of 350 heads. they were ported once before but obviously not professionally b/c the outside edges seem to be thinner than the rest of the ports. I told him that I would port them until they were square again (flush with the edges that were already ported) but he's worried that he might be getting too thin if he did that, so he wants to know the specs on what is normally allowable.

i should maybe have explained it a bit better, the walls between the ports are thinner at the edges where it goes up against the intake than they are thru the rest of the ports.

just gasket match the ports but make sure the intake ports are the same size or you wont help anything, when the air hits the heads it will be like a brick wall and hurt airflow.