Will a 1984 Chevy 305 4bbl that came out of a Monte Carlo fit in a 1978 Monte Carlo that had a 305 2bbl?

It will work, but you would need a 4bbl intake or an adapter plate to fit the 4bbl carb on a 2bbl intake. There are many 4bbl intakes for small block Chevy's available in the aftermarket, or even the junkyard. I would recommend just getting a different intake instead of adding an adapter plate due to possible clearance issues between the hood and the air cleaner.

yes it dousent matter what the carb sizes are when it comes to the same size of motor for space the only problem you should have is that the headers may need to be replaced for spaceing but for other jobs like this a 302 block is the exact same size as a 400 block jsut bored out alot more