There are no specific headers made for an Impala (you are kidding yourself if you think manufactures build them for such a small market). What the manufactures do is build a set of headers by hand and then after the pipes are bent by robots and then welded together by robots and painted by robots they test fit them into a STOCK
car body by bolting them onto a STOCK
motor. By STOCK
I mean using the factory motor mounts and stands to bolt in your non-factory installed motor with the factory available transmission options using the factory linkage at a factory specified ride height using stock wheels and factory specified tire sizes (as all three affect ground clearance if you change any of the above three from stock).
If the motor fits it is listed in the catalog as being just great for your car. If it doesn't fit they still list it in the catalog as fitting your car, but put a foot note telling you what the problem is (what it hit, or what factory options won’t work with their headers such as A/C). So read the foot notes.
The only way to get a custom fit for a custom application is by buying a custom header from Lemon's or any other custom header manufacturer (he has "custom" headers sitting on his shelf ready to deliver to you if your custom one of a kind car is just like all of the other custom one of a kind cars that has a problem with header fitment because of aftermarket parts (you have to list every part you have on the car to get a set of his headers or ship the car to his shop)). But odds are you are not the first person to lower your car and stick wide oversized tires on it with aftermarket angle plug raised port aluminum heads.
Well there is one other way to get custom headers and it is what I generally duo with my cars as I like them to be able to both breathe and to be very, very quiet as I am hunting unsuspecting car nuts who think their car is faster than mine. He he he (as in Elmer Fudd hunting wabbits). That is to buy from your favorite header manufacturer a custom header kit. They are cheap and easy to use.
A custom header kit consist of two header flanges and a box full of bent tubes in the shape of 45, 90. and 180 degree bends. The lengths of the pipes can be added to with straight tubing found at your local auto parts store or you can do what I do and take it a muffler shop that specializes in hot rods or loud fast Harley Bikes (you will have better luck finding the biker working in the muffler shop than say Chip Foose). In Tampa, a medium sized city, there are three shops that I have used in the past to make a custom exhaust for seven different cars. It cost me the same as a set of headers bought from Lemons (about $2,000) but it is gas tight and fully welded from the header collector flange to the stainless steel Bow Tie exhaust tip which is my trade mark of a custom built car.