I've just finished the build out of my own IM lagoon 50 EXT so feel free to ask any questions you may have.
With that said, here is what I've done (sorry for the garbage pictures)
(dry fitting so please excuse the cockeyed gate valve and short emergency drain)
From the overflow box - For the primary drain, I have a have a union directly below the overflow, a longer piece of pipe that goes straight down to a *45, then the gate valve, another *45 and into the sump. I placed the gate valve down by the sump because I'll want ready access to it, rather then trying to get behind the tank. For the emergency drain I have another union directly below the overflow, a long straight piece of pipe, another union, a 90*, a small piece of straight pipe, another 90*, more straight pipe, a 3rd 90* and a piece of pipe about 10" long that sits just below the water level in the sump. In the 2nd image you can see how I routed the emergency to the other side of the main drain.
For the return - A piece of flexible hose to a barbed fitting, a T going to the manifold, a 45* then a union, a run of straight pipe, another union then a 90* into the return bulkhead.
I only used slip fittings (nothing threaded) and used 1" for the drains, 3/4" for the return and 1/2" for the manifold.
Thats the quick and dirty version, let me know if you have questions.