While you are in there verify the jet sizes. I had a slant 6 with a 1bbl Holley that got horrible mileage and ran poorly. It had a reman carb on it when I bought it and I put another reman on it with no change. After digging thru the manual I could on conclude the jetting was off so I cracked the warranty seal and sure enough it had a #664 jet were the book called for a #50. Bought the right jet, put it in and it made everything alright.

I looked at my go to carb parts source and they don't show that check valve though in other applications I have seen a check ball used when a check valve was shown. That might be an option for you.

https://carburetion.walkerproducts.com/holley/?