so you want to drill the 150?
have you read any of bean failsafe overflow thread?
that is a great place to start…
if it were me i would drill three holes along the back across the top (all three holes along the top at the same level)
now it depends how you want to do this…do you want an internal overflow taking up approximately 5" front to back and 7" tall all the way along the back of the tank? or do you want to build an external overflow box and only lose about 1.5" front to back? you are going to lose 6-7" of height regardless of which way you decide to go…it is only the front to back space that will change…
you need more glass cut for the external combo, but it is a slightly cleaner look, (cleanest being the tank notched for a full length overflow box prior to assembly) or you get a couple pieces cut and setup a full length internal box, which is much easier for first timers with silicone and this type of work….as luck would have it if you are sucky with silicone the external box will be ever so non forgiving…where as the internal box has no chance of leaking…
as for hole location, the holes can basically be anywhere along the top edge that is 2 diameters away from any edge…if drilling for 1.5" bulkheads, you need 2.25" holes therefor the centres of the holes need be minimum 4.5" down from the top edge of the tank and 4.5" apart from one and other minimum, the further apart the less chance of cracking. from there you get glass cut to what ever size gives you enough room to spin out bulkheads and 90* fittings…the fittings in the boxes can be slip and don't need to be glued…so figure out how far down a downy turned elbow will be in one of the holes add~ 1/2" for water to get into said down turned elbow and measure back to the top of the tank, then subtract 1.25" for wiggle room (fluctuating water levels in the display) and that is how tall your first piece of glass needs be, then measure the internal dimension of the tank less 3/16" to get the size of the baffle, then you simply need to figure out how far into the tank the fitting will stick, plus the room to back it out plus the room you need to get your hand in there, and that measurement will be the bottom piece at the same length as the baffle…
you have to take in account eurobracing and cross braces as well, and whether they will interfere with installation or maintenance and make the box big enough to accommodate that…
from there the plumbing is pretty straight forward in terms of draining the tank…to return water back go up the back side and over the top of the overflow box back into the display, keep the tips below the top of the water to prevent pushing dirty protein laden water back into the water column.
