Most aftermarket heads, and even many ported stockers have rather large exhaust outlets. The headers used with these usually need ground out to match the heads. Mine needed ground out so much, I ended up welding a bead all the way around the port to make sure I wouldn't grind through. Just be sure to leave clearance for the bolts.
About the only way to get around this, is to purchase large tube headers that use adapter plates that bolt to the head. These plates bolt to the head with countersunk allen screws, then the head bolts to this plate with a larger bolt pattern. These plates allow for much larger primary tubes. You still get into grinding, though. The plates usually have undersized ports, since it is understood the installer will be grinding them out to match whatever head he will be using.
However, for 95% of the applications out there, simply grinding out the headers to match the heads is all that's needed. Make sure to grind them out plenty large enough, since the header will shift up, down, left or right as it's bolted up. It's also important to blend the 'port matching' down into the header as far as possible.
Placing a header gasket against the head, then the header will give an idea of how bad the mismatch is. As long as the header is larger than the head, all is good.
Good Luck!