i just got done with a refuge project, but i messed it up.... first your overflow will drain as fast as your return pumps.... it would be easiest to have your fuge and sump all in one, what i would do is have it drain to a wet/dry... with that you would have water from overflow drain to your sump with egg grates glue different levels of shelving for media, then put up a wall to section off part of the sump, i would get a piece of acrylic, glue it to the top of your 55, and leave about an inch to two inches at the bottom of the tank.. then you can put live rock, and then put up another wall about half way up the tank so water overflows that wall, then you can have your sand bed and plants/algae... then put up another wall so water over flows it and you can have your skimmer and return pump
if you still want a sump and pump, have your refuge built where your overflow drains to the bottom of the tank (you would have to get both ends of the tank drilled, at the bottom) put your sand and media, then it would drain to your sump where you could put your skimmer and return pump... i just went through all this last week | 
