Motor won’t stay running

I have a few thoughts:
1. I am not trying to be rude, but when you completely rule out something, in this instance the carb, you can chase your tail forever looking for other problems only to eventually find that it is the carb. I have learned that the hard way! I am NOT saying it is the carb, I am just saying not to rule it out at this stage. It's like losing your keys and saying, "They couldn't possibly be in the garage, so I won't even look there. Then, two days later, you fine them in the garage. Oh yes, I remember now. My phone rang and I set my keys down so I could dig my phone out of my pocket!
2. You said the carb worked fine on another 400 big block 3 weeks ago. Perhaps something got jostled or tweaked between engines. Perhaps the idle adjustment for one engine isn't working well on the other. Perhaps a float is sticking. Who knows, but I would not rule out the carb yet. You've got several very knowledgeable guys suggesting that it could be the problem. Try this; crank the mechanical engine idle speed screw (the one on the linkage, not the idle mixture screws) in so that when the engine starts, it will run at way above an idle speed. That should help bypass the idle circuit.
3. Get someone to help you by dribbling some gas down the carb when it starts. If it keeps running then but not on it's own, I would lean towards carb, fuel pump or a partially clogged fuel filter.
4. It is not on an actual Engine run stand, so perhaps something is not hooked up properly? My friend Mark, the guy with the Studebaker Power Hawk, has an actual engine run stand. We have had several engines on it. It sounds to me like your setup is not working well for you if you can't even get it started and then shoot some gas down the cab before it dies.
5. Bad timing could be your culprit. That would be tops on my list.
6. A vacuum leak will ABSOLUTELY allow an engine to start and run but NOT idle. But that will be hard to check if you can't get the engine to stay running.
7. You might have to install the engine to figure this out. That way, everything can be hooked up properly, and you should be able to figure the problem out easier.