If you cap it, you will be eliminating your fuel tank vent. You will then be creating a vacuum in your tank as you drive the car and the fuel level lowers, which can cause vapor lock, and can also cause the tank to slightly flex as the vacuum becomes stronger. I had my Cuda set up like this for a while, and when I would take the gas cap off to refuel you could hear a "woooosh" as the air rushed back into the tank through filler tube, and you could also hear the tank flexing back into it's original position. But, if you do plug the line, a vented gas cap is all you need to solve this problem - that's what I ended up doing.

You could also just make some kind of a small breather/filter for the end of the line to keep crud from getting in it, and leave it as-is. As long as the filter doesn't block air from flowing in and out of the line, you should be fine, and the line will remain as a tank vent.

Connecting it to the carb vent may work fine, but I'm not sure. It may work fine as far as venting the tank, but the carb may not like it. You might be creating another problem to solve the first. My guess is that you would be connecting two things together that both need to be vented, but not have an actual vent for either one, giving you two problems instead of one.