You can't leave that like that - the CPS will misread something terrible. The reason the factory ones are mounted in an aluminium boss is that ally is non-ferrous and non magnetic so it won't interfere with the sensor. If you mount it directly on iron the block itself will get picked up and interfere with the flywheel reading.

Better to enlarge the hole while the engine's out. You've got another block to compare it to as well which should make it much easier.