You can call Brodix direct to get specific advice on drilling your Brodix heads corrrectly for 400SB steam holes.
When I did my 406, with vortec heads, I did not drill for the 400 steam hole.
Instead i simply connected the water pump bypass hose to the water port outlet on the drivers side rear of the block (by the distributor). This is your heater hose. It most be connected heater core or not.
Connecting it at the back of the intake manifold instead of at the front, near the thermostat really helps the colling system function. Eliminates all thermostat open close fluctuation and resulting water temperature spiking on any SBC during warm up and idling. and your 400SBC. My 406 never overheats.
If your intake manifold has the extra water ports at the rear, by the distributor:
I highly recommend you do this simple mod even if you do end up drilling the steam holes in your heads.
many intake manifolds have the extra water ports on the rear of the intake manifold just for this improved cooling system function. GM originally routed the water pump hose just like this on old 60's era engines prior to the emissions era, when they then wanted to the make the chevy engine generally run hotter in an attempt to aid emissions control at the time.
Its pretty simple and it works.