detonation / knock issue on 340
That's a model 4150 with 1 9/16 primary and secondary throttles
From an older numberical list, the Throttle body and shaft assembly is (was) p/n 12R11086A
Don't think its available new. Could check for new old stock on ebay or look for a cheap used one, or carb with one.
I don't recommend paying stupid money for a new carb unless you're racing. Find an old one at reasonable price. If you can find another baseplate for your 600 double pumper, it should be easy to get a nice crisp throttle response. Yes it may be down a few mph in the quarter mile compared to a well tuned larger carb, especially with headers and gearing. Doesn't seem like thats your focus right now.
It may be easier to find another 4776, or a 4777 to get you going for now.
