When you go into the blueprint editor, you can see what is happening to the variable, when say, you type in 3 billion for the default value, the editor is resetting it to the Maximum Negative value. This is ocurring because the maximum positive value has been exceeded, by flipping on the high order bit of the 32 bit value. So now that you know the maximum value, you can either try to stay under that value.
Or, you can try to deal with it, and create a 64 bit value, and attempt to work with that. I cannot think of a way to easily do this in blueprints. You will be much better off, doing this kind of work, in C++ as a plugin, and setting up blueprint entry points into the plugin, for the arithmetic that you need. Probably by creating a structure, to represent the value, as you will need to keep the value as C++ needs it (so your not converting back and forth all the time), and as the blueprint needs it (i.e. 2 integers).
Hope, this helps,