As mentioned, the easiest way to clean it is with a shopvac nozzle sucking the particles up as you go.

The heat crossover heats the manifold so that the divorced choke will operate properly (in concert with the heat riser in the exhaust manifold) and it also helps keep the carb warm to atomize fuel better.

From my experience, even if the engine will start/run with a blocked crossover, you’re still losing MPG and washing your cylinders with fuel due to a choke that might not open fully. The effects of this vary with the climate you live in.

It’s also been my experience that this is a smallblock issue. I’ve had two 318s do this to me (one did it twice) but I’ve not yet had (or seen) a bigblock plug this passage.

Blocking/not blocking the crossover can be a heated discussion (pun intended) but IMO the closer to stock the engine is, the better off you are to maintain its function.