Post a pic of the crack on the block where the oil feed line attatches to, then you can know your options. There can be lots of pressure there at high RPM, but it might not leak at idle since idle is roughly 10 Psi, while full redline is closer to 60Psi, which is a lot higher. So I would try and get rid of any crack formation and then plug it off and forget about using i, too risky to loosen it and remove it again in the future, plus weight of the line and vibration might cause it to leak again.