I personally don't use overflow boxes; a bulkhead installed up near the top of the tank, and a straight run of pipe down from that to the sump is my effective, simple and cheap method. It may be too noisy for some who prefer one of the complicated drains designed to minimize the gurgle of water, but it works beautifully and can be muffled very well if desired. Mounting a T fitting on the outside of the bulkhead, with or without a very short section of open pipe pointing upwards, acts as a breather and allows you insert something like a very loose-fitting brush into the top of the pipe, which will cut down on the gurgle to a large extent.
Plenty of charts online that indicate flowrates in various pump sizes; this will be a non-pressurized gravity-powered drain, which flows much less than any pressurized set-up, but I still see numbers up over 3000gph in various places. My largest plywood tank has a realistic flow rate through the drains of about 2400gph when both pumps are operating; plugging one of the drains experimentally shows that the other one all by itself will not quite handle the flow, but with both open I have a ridiculously large safety margin. If I had an actual flow rate of 5000gph...as opposed to reading the number off the box and ignoring head pressure losses, etc...I'd likely use four 2-inch drains and bask in the knowledge that it was way more than needed.
I have a 90-degree elbow on the inside of the bulkheads, mounted on a union that allows me to rotate the elbow which gives me a useful amount of adjustment of the ultimate water level in the running system. Turning the elbows so they face upwards, versus facing them sideways (or removing them altogether), will make a difference of about 2 inches in water level.
Multiple bulkheads means that each individual one is handling a smaller portion of the overall flow rate, which is useful because there is less "draindown" of the main tank during a power outage or when the pump is turned off for maintenance. Less draindown means it is easier to set up your sump with sufficient storage volume to prevent a mess on the floor. And of course with more drains you are safer from mishaps caused by accidental partial or complete blockage of one of them.
If you decide to install a vertical pipe on the inside of the bulkhead to reach down to the bottom, you will want to swiss-cheese it with drilled holes from top to bottom; too few holes, and/or holes that are too small, will eventually slow the flow rate as the holes gradually become clogged. This is what I do with drains for most tanks, but honestly I don't see much difference in water clarity or in buildup of waste matter on the bottom of the tank. The more "stuff" you have hanging off the bulkhead inside of the tank, the greater the risk of a turtle or large fish messing with it and breaking it or causing other problems.
My first plywood tank had bottom-drilled holes, with vertical standpipes screwed into the bulkheads and reaching right to the surface. I thought I was soooo smart doing that; the logic was that I could change the height of the verticals and thus the water level in the tank. Unfortunately, a big-ass critter jamming itself into the gap between standpipe and tank wall allows the beast to exert sufficient side pressure at the top of the pipe, that it can be cracked at the bottom! Don't ask...
