As Dave has noted your hood has a unique Hollander Interchange number. So even though 1965 through 1970 Chevy B-bodies all shared similar parts and will bolt on to each other, there is enough difference (I'm guessing at the front of the hood where it over hangs the grill) that is different to make this a one of a kind hood.
The Hollander Interchange Manual is considered to be the definitive source of what will or will not fit. Within the manual you will find a listing of every part on the car from the cigarette lighter to the doors, light bulbs to motor will all have assigned to it a Hollander Interchange number. The book lists the complete parts list of every domestic car and light truck made in the US for that year (each edition covers seven to ten years back depending upon how many donor vehicles shared that common part).
Hollander took every new car and truck apart to see what changed from year to year and they compared the interchange fitment of every part before you ever attempted it. If it was a direct interchange it retained the old part number from the previous year.
If it fit, but it had some extra bit hanging off, or it had some additional holes drilled in it, that an older vehicle didn't need, and the change wouldn't interfere with the use of the part on an older car, it got the old number with a letter added after it (the letter was an "A" I believe, but I would have to drag my Hollander off the shelf to be sure).
If the part would fit but the added bit had to be modified slightly to be used (such as sawing off an added ear or drilling an additional hole) it retained the old part number with another letter after it (I believe that part got a "C" after it). Using this system you KNOW what will or will not interchange, and just how hard it would be to use that part to restore a car to new condition. (Keep in mind the customer's of these books are junk yards selling to auto body shops doing collision repairs).
Knowing this you can take the Hollander Interchange number Dave supplied you with (786) down to your local salvage yard (or recycler if that is what they call themselves instead of junkyard) and have them put it on the "LINE". The line is a network of every yard in the Canada the US and Mexico. If it exists anywhere for sale in any yard on the North American continent, you will get an offer to sell back with price and condition as well as location. Shipping something like a hood is going to be by truck, so it's going to cost as much if not more than the hood to get it to you.
There are a lot of parts such as the 1969 hood that fits 90% of the way but the parts are unique enough to get it's own interchange number. It doesn't mean that if this was the last hood on the planet that you couldn't take a 1965-'70 hood and fabricate all new metal to make it fit; it just means it is more work than Hollander thought would be economically feasible for a body shop to use that part back in 1969.
By the same token you may find a Ford or a Chrysler Jeep as a source for an interchange part (such as a Saginaw steering gear box). That isn't a misprint. It actually fits. This is because in this case, Saginaw gear division of GM sold Chrysler and Ford Saginaw gear boxes. It was cheaper for Ford and Chrysler to buy than in making their own part.
Similarly You may find some other odd ball car or truck being a donor vehicle. This is because GM bought a part from an outside vendor; such as a Dana-Spicer rear end (the old 8.2 inch ten bolt or the twelve bolt rear ends). These were also sold to other vendors for use on their vehicles. If the interchange number says it fits it will fit, but it will exclude a lot of parts that you might want to consider as an interchange part because what was too hard to do in 1969 is your only source of a part today. This is why you need a newer book to find parts off newer cars that will also fit your older car.
The Hollander Interchange Manual is a leatherette bound series of two to four paper back books that are gathered together in one enormous volume. The book is segregated into individual sections (such as doors), and it will list every door for every domestic vehicle made that year (imports had their own Hollander Interchange book, but they where not a big part of the salvage industry back in the sixties). Because every part made over ten years time makes for a lot of part numbers these books are huge (four to eight inches thick). And they included every part made from cigarette lighter and windshield wipers to trunks and quarter panels.
The Auto Truck
Hollander Interchange Manual you want is the 32nd
Edition. It covers not only muscle cars such as your Impala, but every car and light truck made in 1969, and goes back in time about seven years to provide you with other options for donor cars besides the 1969 Impala you know already fits. You should know that all B-body cars made by GM (and some C and D body cars as well) in 1969 will also fit.
Because the 32nd Edition stops at 1969 you will also need a newer book published about seven years later to find any additional parts that still might interchange. (Keep in mind Chevy used the same water pump gasket from 1955 through 2004, even though there are three different water pumps made to use that same gasket). There may be parts off of a 1999 Caprice that will still bolt up to your 1966 Impala (such as the starter motor). If it will fit it will still have that same Hollander Interchange number, regardless of what year it was made.
Another way to obtain a listing of Hollander Interchange numbers is through a reprint. Hollander has published a condensed (missing 80% of the information) version of their old manuals that covers a range of years. You will be interested in one of these:
Hollander Interchange Classic Search Manual III
Format: 2 volumes, 1,115 pages
Coverage: 1954 - 1966
Hollander Interchange Classic Search Manual IV
Format: 2 volumes
Part A: ( ISBN: 1-58132-155-4) 1786
Part B: ( ISBN: 1-58132-156-2) 1664
Coverage: 1965 - 1979
Hollander also sells a model specific collection of condensed interchange numbers for the Corvette, Camaro, The Nova and the Chevelle as well as the Ford Rustang if you have one in the garage in need of restoration.
You can often find these books used for a lot less on Amazon dot com.