I ran a 10g sump on my 72 for a long time. It can be done. As Fiji stated you will have to top off on a regular basis. I have a 30g sump just for my skimmer and my return pumps and I have to top off about 3g per day now.
As to how to get the water out of the tank and down to the sump you will need a hang on the back overflow box. They are a dime a dozen online. Check on craigslist and ebay. The overflow will take water from the tank and drain it down to the sump. A pump in the sump will pump the water back up to the tank starting the circle over again.
Have a look at this
, Marc is very good at explaining a sump and how to set one up.