I have done a couple of the repro headlights and I found that a dremel and a load of bits work the best. You will have to shave down the base also.
What happen is when they make the part, they use a mold from the original, but in the casting process the metal shrinks smaller then the original. This is a natural thing.
When your done grinding it down, bring it to a chrome shop and have them polish it, since it's made of zinc and tin it will polish to a chrome like shine. Then just put a clear on it. That's what I did with this one.
Untitled by
then8j, on Flickr