I took a look at it. Yes it already has a spacer but wasn't sure if it was aluminum or plastic due to all the fuel stains from where the carburetor boils over. The intake manifold has a nice piss yellow stain on both sides below the carburetor. Probably from the corn used to make the ethanol...

Generally, its thought that the fuel should stay inside the carburetor. Leaking isn't good! The phenolic spacer can crack. Generally as a result of over tightening the flange nuts. The flange nuts can come loose, and two of them are difficult to get at. Hence people over tightening them to make sure that stay done up, and resulting in cracking the spacer. It is feasible that the residue is from spray testing for air leaks.

I don't smell any fuel while the car is running. I just assumed it's from where fuel boils over after I shut the engine off. Everyone of my carburated cars has this nasty yellow stain on the intake manifold.

I've been meaning to pop the bonnet and observe what happens after shutting the engine off but I figured with the bonnet up, this would release all the heat and nothing would happen.

Yes fuel should stay in the carburetor but the fuels these cars were designed to run on is not the same as today. They say they are formulated to burn better in fuel injected engines. I don't know if I understand why the fuel would need to be any different for it to explode in an engine.

I will pull the carb and check the spacer and see if it's cracked.

Best just to check for air leaks. I use ether myself, because there is usually some handy. Some use petrol, some butane. With the motor warmed up and running, I spray a little around the suspect area. If it alters the rpm, then that is usually a sign.

Not blowing your self up is preferable.