Off the shelf headers are universal fit headers. That means they will universally fit nothing.
If you look up the part number for the header you are looking to buy you will discover that the manufacturer says that same header part number fits every SBC (for a SB header), or BBC in every car that Chevrolet made over the range of the series. In other words your BBC Impala header will also fit a Camaro, Nova, and Chevelle body equally well from 1968 through '90. They don't list them for light trucks as they will hit the firewall (car headers do not drop down low enough to fit a light truck), and they won't fit a Corvette as there is just no room under the hood of a Corvette for most off the shelf headers.
To get a guaranteed fit you have to make your own from a header assembly kit, or to buy custom made headers from an old school muffler and exhaust shop just like the Jerry Stahl, or Bob Headman, or Doug Thorely originally did in their muffler shops offered back in the early sixties.
Lemon's will custon make a standard shaped header for you as he claims to be a custom fabricator but he fits one car with a specific set of heads and makes all others of that body type to fit the pattern if they share the same head.
If he has never seen your combination before he will custom fit tubes to your car for about $3,600 but only if you get your car to his shop. Then every one else who has your car and motor combo gets the same shaped headers for only $1300 made from your new pattern he established.
Today's headers are all assembled from robotically cut and bent pipe by a Mexican (he could be a Korean for all I know, but the headers are made in Mexico) in a jig and another robot welds them together. Then the low wage per hour worker (still think he is probably a Mexican) takes the header from the welding robot and puts it in a box after it is either painted or coated by another robot. Headers are not equal length or even designed for performance just appearance (show instead of go) as most don't care about performance, being more concerned about how loud their exhaust is, and how lumpy their cam is.
By the way don't put in a 427. Rebuild it with a 496 rotating assembly to get the same level of power out of a pump gas motor (your 427 will not run on 11:1 compression that the 390 horse Impala 427 motor had back when your car was new). I used to get 550 horse or more out of a high winding L-88 427 at the drag strip that ran on Sunoco 260 back in the sixties, but you can not buy Sunoco 260 at the pump any more as today it comes in a five gallon pail for $58.99 ($11.88 per gallon).