Depends on where you want your overflow. But rather than drilling 1/2 down the tank, just drill a few inches down and do an external durso rather than the internal and then your overflow box does not have to stick out into the tank as far, plus if for any reason the box fails, you will not drain the tank as low. In your setup if your overflow box failed, you could drain the tank down 1/2 way, as low as the bulkhead, if the bulkhead is higher up in the tank it could only drain as low as the bulkhead if things went very wrong.
On a 4 foot long 75, I would either suggest putting the overflow in one corner (much simpler design or doing a califo style long overflow, a large center overflow will take up a lot of space in the tank.
kim
