I use slip fittings for my bulkheads. Although I have only used one external overflow box before, I didn't think they used bulkheads? I was under the impression the flex hoses slip into the overflows and attach somehow? If you are using a bulkhead I would go the slip route and glue it in.
well according to the place I was thinking of ordering the overflow box from says its a standard bulkhead but is not a standard thread on them and was told I wont find anything to fit it ?

