All well caps that I have ever seen are not there to prevent overflowing, in fact they are vented so the well can breath. We have a high pressure (12 ft of + head)flowing well at our home, and it has a seal down about 7 feet in the casing. I have always done my own pump work, if the pump fails, I plan on loosening the 4 set screws and try pulling it. this photo is looking into the top of the casing. I will put a container around the casing and pipe it away from the well, because there is going to be a lot of water coming out once I pull the seal out of the casing. You must live in a warm area if the well can be allowed to overflow.
We have another property where we also have a flowing well. I made up a casing extension with a 2 inch pipe welded onto the extension just above the no-hub coupling and the water is piped away from there. I don't have a picture of that.