I would personally go with Willamette as the finishing hop on a stout. For 5 gallons, that seems like a lot of cacao also. Make sure you soak it in something to sanitize it (100 proof rum is my personal pick).
+1 for the tincture and +1 for the 1450 and +1 for Willamette would go great in a sweet stout. I don't think 5 oz is too much depending on how you use it. I like the 1:1 ratio on the oz/gallon of knibs. They are a subtle chocolate flavor, and give off more aroma of chocolate to me anyway.
For your recipe I would back off the RB to 5% and keep the chocolate to 8.5% for a 13.5% dark malt. I like to use Midnight Wheat for a 1-4% of the grist within that 13.5% dark malt. Well when I can find midnight wheat. Not as easy as I like but it isn't terrible either.
What Chocolate character are you looking for? Two very different but available examples IMO that taste great. One is a stout and One is a Porter so obviously there is a taste difference there, however the approach on getting the chocolate is different two, with apparently different results. Depends which direction you want to take your beer.
Youngs - Double Chocolate Stout [Dark Chocolate Bar and Cocao Knibs in BK, Chocolate Malt -Chocolate Cake in a glass] Bittered with Fuggle and Goldings
Boulder Beer - Shake Chocolate Porter [Cocao Knibs for aroma with chocolate wheat, chocolate malt, and black malt - Velvety soft chocolate flavor, with an aroma and overall flavor of rich bitter chocolate and tastes like a chocolate malt shake] Bittered with Tettnang and Nugget
So what are you looking for in Aroma, or Texture? A pronounced sweet chocolate flavor, or nuanced bitter dry chocolate flavor?
IME Cocao knibs give a nuanced chocolate aroma, and dark chocolate flavor on the back of my tongue. Dark Bitter Chocolate gives a very smooth milk stout like texture. So thicker, overly full body, and loads more flavor but aroma dies down while the beer is still green.
My favorite blend on a sweet stout was Cocao Knibs in the BK with 2.4oz/1Gallon of Dark Chocolate Bar at F.O. (chopped up very fine to melt quickly during chill) and Black Rum [not the spiced Captian or Jamacian stuff - The Black Dutch Carrabien kind] on 1.04 oz of Cocao knibs /1 gallon of beer with enough rum to cover completely. I held it in the fridge for 7 days then pitched the whole thing in while I racked to a bottling bucket with my Priming Sugar.
In case you are wondering why on the Black Dutch Rum - It is going in the beer too. And the carried over flavor is not going to be noticed in the final product. But it generally has higher ABV than the white crap but is mellowed out from being barrel aged and doesn't have a jet fuel after burn. I am afraid of putting 151 proof white rum in my beer because it tastes like crap, but a 140 proof Charred Oak Barrel Aged Rum is on my shelf because it tastes GREAT. And the 151 crap isn't. So call me bias...
And I will say you will pull out the essentials from the Knibs using 100 proof rum or vodka. But the black rum is roughly 4$ more per 750ml bottle, and you will have plenty left over. And unless you have a rum runner in your future I wouldn't advise anyone buying 100+ proof rum, cause I hate it.
like I was shy about that any point during that rant... Cheers!
**Edit - if you are looking for strong coffee with it, I like to put high quality espresso in my bfast coffee stouts 1 shot/1gallon too. But I would not do that with a rich chocolate bar stout, but idk what you are after