So you are going to be building a coast to coast overflow? What size tank are you doing this too? Have you looked at the glass holes overflow kits?
http://www.glass-holes.com/Overflow-Kits_c3.htm
In my opinion you are better served not worrying about the returns to take care of the flow, rather rely on power heads to cover the flow. However if you are relying on your returns to supply all the flow to the entire tank then I assume you are not planning to use the sump/refugium in the traditional manner of housing DSB (Deep Sand Beds), Macro Algaes or a skimmer as these things require low flow and you will need high flow through the sump area if you plan for the overflows to be the only means of flow in the DT (Display Tank).
* I agree with Zombie though you need to have some sort of siphon breaks in order to stop sump flooding when the power fails or return pump fails (its not if its when). If you plan to use a check valves in addition to the siphon breaks, I would recommend unions on both sides of it so that you can take out regularly and clean them.