I really like garden eels and did a lot of asking and reading about them a while back. You might be able to get away with the tank size but you would probably need a bit deeper sand bed. The problem is really going to be feeding as innovator mentioned. In the wild they feel slowly all day and this is very difficult to reproduce in the home aquarium for several reasons. First, they are not like fish and won't go to the food. The food has to go to them and this is made much more diffucult by the fact that they hide when you get close to the tank. Second, feeding that much in almost any home aquarium is going to take some really heavy duty nutrieunt export. Far beyond just an oversized skimmer and extra water changes. Third, just from the point of being practical it would be a huge time commitment to feed that often and prepair their food (which needs to be finely chopped)
I know how it is since i have gotten several difficult to keep critters stuck in my head including garden eels. These are truly a species that is going to be extreemly difficult to provide for. I think in the long run you will get much more enjoyment out of a mixed reef with multiple fish/inverts/coral then you would out of just a species tank of garden eels even if you were able to care for them. It's difficult since they are such facinating little guys but I had to self this idea too.