Minimum Clearances when drilling an overflow

[ensquire]

My overflow has 2 holes right now and I'm gonna do a Herbie, but space is tight. It currently has a main drain thru a 1" bulkead and a 1/2 bulkhead for the emergency overflow. I tested it running a Quiet one 4000 for the return at about 4' of rise to the head. ran return wide open and had to throttle down drain to maintain siphon. stopped main drain and it wasn't a big enough emergency line.
My options are a 3/4" or a 1' bulkhead . 3/4 might handle it but would be even safer at 1'.
Which bring me to this question. If I drill a 1 3/4" hole in the space available for the 1" bulkead there will only be a little over an inch between the two holes. and only slightly more for the 3/4. IS THIS ENOUGH????