There is no " set" max bore on an X block. They came virgin with a 318 bore of 3.90ish.
I have heard of some that were not safe past 4.06 bore. As i said earlier, the largest bore i have heard of is 4.125. Could there be some that have been bigger..... sure. But it would definately be by far the exception to the rule. Generally anything 4.08 to 4.10 with plenty of wall left is being fortunate.
You have to remember most people using this block arent building a 450-500 horse streeter. They are looking to make 600-750 horse. At those power levels, even an X block needs to have plenty of meat left to be up to the task.